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.

1 files changed, 32 insertions, 28 deletions

diff --git a/src/builder/edit_guild.rs b/src/builder/edit_guild.rs
index c24b9b4..8c7765b 100644
--- a/src/builder/edit_guild.rs
+++ b/src/builder/edit_guild.rs
@@ -23,7 +23,7 @@ impl EditGuild {
     /// valid.
     ///
     /// [`afk_timeout`]: #method.afk_timeout
-    pub fn afk_channel<C: Into<ChannelId>>(mut self, channel: Option<C>) -> Self {
+    pub fn afk_channel<C: Into<ChannelId>>(&mut self, channel: Option<C>) {
         self.0.insert(
             "afk_channel_id",
             match channel {
@@ -31,21 +31,17 @@ impl EditGuild {
                 None => Value::Null,
             },
         );
-
-        self
     }
 
     /// Set the amount of time a user is to be moved to the AFK channel -
     /// configured via [`afk_channel`] - after being AFK.
     ///
     /// [`afk_channel`]: #method.afk_channel
-    pub fn afk_timeout(mut self, timeout: u64) -> Self {
+    pub fn afk_timeout(&mut self, timeout: u64) {
         self.0.insert(
             "afk_timeout",
             Value::Number(Number::from(timeout)),
         );
-
-        self
     }
 
     /// Set the icon of the guild. Pass `None` to remove the icon.
@@ -67,7 +63,11 @@ impl EditGuild {
     ///
     /// let base64_icon = utils::read_image("./guild_icon.png")?;
     ///
-    /// guild.edit(|g| g.icon(Some(&base64_icon)))?;
+    /// guild.edit(|mut g| {
+    ///     g.icon(Some(&base64_icon));
+    ///
+    ///     g
+    /// })?;
     /// #     Ok(())
     /// # }
     /// #
@@ -77,32 +77,26 @@ impl EditGuild {
     /// ```
     ///
     /// [`utils::read_image`]: ../utils/fn.read_image.html
-    pub fn icon(mut self, icon: Option<&str>) -> Self {
+    pub fn icon(&mut self, icon: Option<&str>) {
         self.0.insert(
             "icon",
             icon.map_or_else(|| Value::Null, |x| Value::String(x.to_string())),
         );
-
-        self
     }
 
     /// Set the name of the guild.
     ///
     /// **Note**: Must be between (and including) 2-100 chracters.
-    pub fn name(mut self, name: &str) -> Self {
+    pub fn name(&mut self, name: &str) {
         self.0.insert("name", Value::String(name.to_string()));
-
-        self
     }
 
     /// Transfers the ownership of the guild to another user by Id.
     ///
     /// **Note**: The current user must be the owner of the guild.
-    pub fn owner<U: Into<UserId>>(mut self, user_id: U) -> Self {
+    pub fn owner<U: Into<UserId>>(&mut self, user_id: U) {
         let id = Value::Number(Number::from(user_id.into().0));
         self.0.insert("owner_id", id);
-
-        self
     }
 
     /// Set the voice region of the server.
@@ -121,7 +115,11 @@ impl EditGuild {
     ///
     /// // assuming a `guild` has already been bound
     ///
-    /// guild.edit(|g| g.region(Region::UsWest))?;
+    /// guild.edit(|mut g| {
+    ///     g.region(Region::UsWest);
+    ///
+    ///     g
+    /// })?;
     /// #     Ok(())
     /// # }
     /// #
@@ -131,10 +129,8 @@ impl EditGuild {
     /// ```
     ///
     /// [`Region::UsWest`]: ../model/guild/enum.Region.html#variant.UsWest
-    pub fn region(mut self, region: Region) -> Self {
+    pub fn region(&mut self, region: Region) {
         self.0.insert("region", Value::String(region.name().to_string()));
-
-        self
     }
 
     /// Set the splash image of the guild on the invitation page.
@@ -143,11 +139,9 @@ impl EditGuild {
     /// You can check this through a guild's [`features`] list.
     ///
     /// [`features`]: ../model/guild/struct.Guild.html#structfield.features
-    pub fn splash(mut self, splash: Option<&str>) -> Self {
+    pub fn splash(&mut self, splash: Option<&str>) {
         let splash = splash.map_or(Value::Null, |x| Value::String(x.to_string()));
         self.0.insert("splash", splash);
-
-        self
     }
 
     /// Set the verification level of the guild. This can restrict what a
@@ -166,26 +160,36 @@ impl EditGuild {
     ///
     /// // assuming a `guild` has already been bound
     ///
-    /// if let Err(why) = guild.edit(|g| g.verification_level(VerificationLevel::High)) {
+    /// let edit = guild.edit(|mut g| {
+    ///     g.verification_level(VerificationLevel::High);
+    ///
+    ///     g
+    /// });
+    ///
+    /// if let Err(why) = edit {
     ///     println!("Error setting verification level: {:?}", why);
     /// }
     ///
     /// // additionally, you may pass in just an integer of the verification
     /// // level
     ///
-    /// if let Err(why) = guild.edit(|g| g.verification_level(3)) {
+    /// let edit = guild.edit(|mut g| {
+    ///     g.verification_level(3);
+    ///
+    ///     g
+    /// });
+    ///
+    /// if let Err(why) = edit {
     ///     println!("Error setting verification level: {:?}", why);
     /// }
     /// ```
     ///
     /// [`VerificationLevel`]: ../model/guild/enum.VerificationLevel.html
     /// [`VerificationLevel::High`]: ../model/guild/enum.VerificationLevel.html#variant.High
-    pub fn verification_level<V>(mut self, verification_level: V) -> Self
+    pub fn verification_level<V>(&mut self, verification_level: V)
         where V: Into<VerificationLevel> {
         let num = Value::Number(Number::from(verification_level.into().num()));
 
         self.0.insert("verification_level", num);
-
-        self
     }
 }