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.

2 files changed, 9 insertions, 3 deletions

diff --git a/src/model/guild/member.rs b/src/model/guild/member.rs
index 34f2af1..dae2b41 100644
--- a/src/model/guild/member.rs
+++ b/src/model/guild/member.rs
@@ -117,7 +117,8 @@ 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) {
@@ -405,7 +406,8 @@ 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 53239a8..39f6339 100644
--- a/src/model/guild/role.rs
+++ b/src/model/guild/role.rs
@@ -90,7 +90,11 @@ impl Role {
     /// # let role = RoleId(7).to_role_cached().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