Make Guild::create_channel return a GuildChannel

Instead of returning a generic `Channel` enum, make the following
functions return an explicit GuildChannel instead of a more "generic"
Channel enum:

- Guild::create_channel
- GuildId::create_channel
- PartialGuild::create_channel
- rest::create_channel

src/client/rest/mod.rs

@@ -204,14 +204,14 @@ pub fn broadcast_typing(channel_id: u64) -> Result<()> {
 /// [`GuildChannel`]: ../../model/struct.GuildChannel.html
 /// [docs]: https://discordapp.com/developers/docs/resources/guild#create-guild-channel
 /// [Manage Channels]: ../../model/permissions/constant.MANAGE_CHANNELS.html
-pub fn create_channel(guild_id: u64, map: Value) -> Result<Channel> {
+pub fn create_channel(guild_id: u64, map: Value) -> Result<GuildChannel> {
     let body = serde_json::to_string(&map)?;
     let response = request!(Route::GuildsIdChannels(guild_id),
                             post(body),
                             "/guilds/{}/channels",
                             guild_id);
 
-    Channel::decode(serde_json::from_reader(response)?)
+    GuildChannel::decode(serde_json::from_reader(response)?)
 }
 
 /// Creates an emoji in the given [`Guild`] with the given data.

src/model/guild.rs

@@ -296,7 +296,7 @@ impl Guild {
     /// [`Channel`]: struct.Channel.html
     /// [`ClientError::InvalidPermissions`]: ../client/enum.ClientError.html#variant.InvalidPermissions
     /// [Manage Channels]: permissions/constant.MANAGE_CHANNELS.html
-    pub fn create_channel(&mut self, name: &str, kind: ChannelType) -> Result<Channel> {
+    pub fn create_channel(&mut self, name: &str, kind: ChannelType) -> Result<GuildChannel> {
         #[cfg(feature="cache")]
         {
             let req = permissions::MANAGE_CHANNELS;
@@ -1170,7 +1170,7 @@ impl GuildId {
     /// [`GuildChannel`]: struct.GuildChannel.html
     /// [`rest::create_channel`]: ../client/rest/fn.create_channel.html
     /// [Manage Channels]: permissions/constant.MANAGE_CHANNELS.html
-    pub fn create_channel(&self, name: &str, kind: ChannelType) -> Result<Channel> {
+    pub fn create_channel(&self, name: &str, kind: ChannelType) -> Result<GuildChannel> {
         let map = ObjectBuilder::new()
             .insert("name", name)
             .insert("type", kind.name())
@@ -1946,7 +1946,7 @@ impl PartialGuild {
     /// [`rest::create_channel`]: ../client/rest/fn.create_channel.html
     /// [Manage Channels]: permissions/constant.MANAGE_CHANNELS.html
     #[inline]
-    pub fn create_channel(&self, name: &str, kind: ChannelType) -> Result<Channel> {
+    pub fn create_channel(&self, name: &str, kind: ChannelType) -> Result<GuildChannel> {
         self.id.create_channel(name, kind)
     }