Make builders mutably borrowed

Change the builders so that they are now mutably borrowed, accepting
`&mut self` instead of `self`. Their methods now return `()` instead of
`Self`.

Upgrade path:

Change code such as the following:

```rust
channel.send_message(|m| m
    .embed(|e| e
        .description("test")
        .title("title")));
```

to the following style:

```rust
channel.send_message(|mut m| {
    m.embed(|mut e| {
        e.description("test");
        e.title("title");

        e
    });

    m
});
```

Closes #159.

8 files changed, 67 insertions, 28 deletions

diff --git a/src/model/channel/channel_id.rs b/src/model/channel/channel_id.rs
index f6d9963..f4336fc 100644
--- a/src/model/channel/channel_id.rs
+++ b/src/model/channel/channel_id.rs
@@ -402,7 +402,11 @@ impl ChannelId {
     /// [`ModelError::MessageTooLong`]: enum.ModelError.html#variant.MessageTooLong
     #[inline]
     pub fn say<D: ::std::fmt::Display>(&self, content: D) -> Result<Message> {
-        self.send_message(|m| m.content(content))
+        self.send_message(|mut m| {
+            m.content(content);
+
+            m
+        })
     }
 
     /// Sends a file along with optional message contents. The filename _must_
@@ -426,7 +430,11 @@ impl ChannelId {
     ///
     /// let paths = vec!["/path/to/file.jpg", "path/to/file2.jpg"];
     ///
-    /// let _ = channel_id.send_files(paths, |m| m.content("a file"));
+    /// let _ = channel_id.send_files(paths, |mut m| {
+    ///     m.content("a file");
+    ///
+    ///     m
+    /// });
     /// ```
     ///
     /// Send files using `File`:
@@ -442,7 +450,11 @@ impl ChannelId {
     ///
     /// let files = vec![(&f1, "my_file.jpg"), (&f2, "my_file2.jpg")];
     ///
-    /// let _ = channel_id.send_files(files, |m| m.content("a file"));
+    /// let _ = channel_id.send_files(files, |mut m| {
+    ///     m.content("a file");
+    ///
+    ///     m
+    /// });
     /// ```
     ///
     /// # Errors
diff --git a/src/model/channel/embed.rs b/src/model/channel/embed.rs
index 8837a8d..8792d18 100644
--- a/src/model/channel/embed.rs
+++ b/src/model/channel/embed.rs
@@ -80,10 +80,13 @@ impl Embed {
     /// ```rust,no_run
     /// use serenity::model::channel::Embed;
     ///
-    /// let embed = Embed::fake(|e| e
-    ///     .title("Embed title")
-    ///     .description("Making a basic embed")
-    ///     .field("A field", "Has some content.", false));
+    /// let embed = Embed::fake(|mut e| {
+    ///     e.title("Embed title");
+    ///     e.description("Making a basic embed");
+    ///     e.field("A field", "Has some content.", false);
+    ///
+    ///     e
+    /// });
     /// ```
     #[inline]
     pub fn fake<F>(f: F) -> Value
diff --git a/src/model/channel/guild_channel.rs b/src/model/channel/guild_channel.rs
index 533aaf2..c6d9031 100644
--- a/src/model/channel/guild_channel.rs
+++ b/src/model/channel/guild_channel.rs
@@ -496,8 +496,11 @@ impl GuildChannel {
     ///             },
     ///         };
     ///
-    ///         let _ = msg.channel_id.send_files(vec![(&file, "cat.png")], |m|
-    ///             m.content("here's a cat"));
+    ///         let _ = msg.channel_id.send_files(vec![(&file, "cat.png")], |mut m| {
+    ///             m.content("here's a cat");
+    ///
+    ///             m
+    ///         });
     ///     }
     /// }
     ///
diff --git a/src/model/channel/message.rs b/src/model/channel/message.rs
index 978e181..fdc7787 100644
--- a/src/model/channel/message.rs
+++ b/src/model/channel/message.rs
@@ -221,11 +221,11 @@ impl Message {
         let mut builder = EditMessage::default();
 
         if !self.content.is_empty() {
-            builder = builder.content(&self.content);
+            builder.content(&self.content);
         }
 
         if let Some(embed) = self.embeds.get(0) {
-            builder = builder.embed(|_| CreateEmbed::from(embed.clone()));
+            builder.embed(|_| CreateEmbed::from(embed.clone()));
         }
 
         let map = serenity_utils::vecmap_to_json_map(f(builder).0);
diff --git a/src/model/guild/member.rs b/src/model/guild/member.rs
index c95c07f..ce3dd01 100644
--- a/src/model/guild/member.rs
+++ b/src/model/guild/member.rs
@@ -113,7 +113,8 @@ impl Member {
     pub fn add_roles(&mut self, role_ids: &[RoleId]) -> Result<()> {
         self.roles.extend_from_slice(role_ids);
 
-        let builder = EditMember::default().roles(&self.roles);
+        let mut builder = EditMember::default();
+        builder.roles(&self.roles);
         let map = utils::vecmap_to_json_map(builder.0);
 
         match http::edit_member(self.guild_id.0, self.user.read().id.0, &map) {
@@ -395,7 +396,8 @@ impl Member {
     pub fn remove_roles(&mut self, role_ids: &[RoleId]) -> Result<()> {
         self.roles.retain(|r| !role_ids.contains(r));
 
-        let builder = EditMember::default().roles(&self.roles);
+        let mut builder = EditMember::default();
+        builder.roles(&self.roles);
         let map = utils::vecmap_to_json_map(builder.0);
 
         match http::edit_member(self.guild_id.0, self.user.read().id.0, &map) {
diff --git a/src/model/guild/role.rs b/src/model/guild/role.rs
index 53ec478..800d178 100644
--- a/src/model/guild/role.rs
+++ b/src/model/guild/role.rs
@@ -83,7 +83,11 @@ impl Role {
     /// # let role = RoleId(7).find().unwrap();
     /// // assuming a `role` has already been bound
     //
-    /// role.edit(|r| r.hoist(true));
+    /// role.edit(|mut r| {
+    ///     r.hoist(true);
+    ///
+    ///     r
+    /// });
     /// ```
     ///
     /// [`Role`]: struct.Role.html
diff --git a/src/model/user.rs b/src/model/user.rs
index 330f319..131e891 100644
--- a/src/model/user.rs
+++ b/src/model/user.rs
@@ -436,7 +436,13 @@ impl User {
     ///                 url,
     ///             );
     ///
-    ///             match msg.author.direct_message(|m| m.content(&help)) {
+    ///             let dm = msg.author.direct_message(|mut m| {
+    ///                 m.content(&help);
+    ///
+    ///                 m
+    ///             });
+    ///
+    ///             match dm {
     ///                 Ok(_) => {
     ///                     let _ = msg.react('👌');
     ///                 },
diff --git a/src/model/webhook.rs b/src/model/webhook.rs
index ab07b43..8f2c6b2 100644
--- a/src/model/webhook.rs
+++ b/src/model/webhook.rs
@@ -160,7 +160,11 @@ impl Webhook {
     /// let mut webhook = http::get_webhook_with_token(id, token)
     ///     .expect("valid webhook");
     ///
-    /// let _ = webhook.execute(false, |w| w.content("test")).expect("Error executing");
+    /// let _ = webhook.execute(false, |mut w| {
+    ///     w.content("test");
+    ///
+    ///     w
+    /// });
     /// ```
     ///
     /// Execute a webhook with message content of `test`, overriding the
@@ -176,18 +180,23 @@ impl Webhook {
     /// let mut webhook = http::get_webhook_with_token(id, token)
     ///     .expect("valid webhook");
     ///
-    /// let embed = Embed::fake(|e| e
-    ///     .title("Rust's website")
-    ///     .description("Rust is a systems programming language that runs
-    ///                   blazingly fast, prevents segfaults, and guarantees
-    ///                   thread safety.")
-    ///     .url("https://rust-lang.org"));
-    ///
-    /// let _ = webhook.execute(false, |w| w
-    ///     .content("test")
-    ///     .username("serenity")
-    ///     .embeds(vec![embed]))
-    ///     .expect("Error executing");
+    /// let embed = Embed::fake(|mut e| {
+    ///     e.title("Rust's website");
+    ///     e.description("Rust is a systems programming language that runs
+    ///                    blazingly fast, prevents segfaults, and guarantees
+    ///                    thread safety.");
+    ///     e.url("https://rust-lang.org");
+    ///
+    ///     e
+    /// });
+    ///
+    /// let _ = webhook.execute(false, |mut w| {
+    ///     w.content("test");
+    ///     w.username("serenity");
+    ///     w.embeds(vec![embed]);
+    ///
+    ///     w
+    /// });
     /// ```
     #[inline]
     pub fn execute<F: FnOnce(ExecuteWebhook) -> ExecuteWebhook>(&self,