Address some clippy warnings (serenity-rs#924)

* Address warnings for `clippy::option_as_ref_deref`

The warnings were like this:

called `.as_ref().map(String::as_str)` on an Option value. This can be
done more directly by calling `query.as_deref()` instead

* Address warning for `clippy::iter_nth_zero`

The warning was like this:

called `.nth(0)` on a `std::iter::Iterator`

src/client/bridge/gateway/shard_runner.rs

@@ -292,7 +292,7 @@ impl ShardRunner {
                     self.shard.chunk_guilds(
                         guild_ids,
                         limit,
-                        query.as_ref().map(String::as_str),
+                        query.as_deref(),
                     ).is_ok()
                 },
                 ShardClientMessage::Runner(ShardRunnerMessage::Close(code, reason)) => {

src/framework/standard/parse/mod.rs

@@ -24,7 +24,7 @@ fn to_lowercase<'a>(config: &Configuration, s: &'a str) -> Cow<'a, str> {
 ///
 /// [`Configuration::on_mention`]: ../struct.Configuration.html#method.on_mention
 pub fn mention<'a>(stream: &mut Stream<'a>, config: &Configuration) -> Option<&'a str> {
-    let on_mention = config.on_mention.as_ref().map(String::as_str)?;
+    let on_mention = config.on_mention.as_deref()?;
 
     let start = stream.offset();
 

src/model/channel/group.rs

@@ -236,7 +236,7 @@ impl Group {
         match self.name {
             Some(ref name) => Cow::Borrowed(name),
             None => {
-                let mut name = match self.recipients.values().nth(0) {
+                let mut name = match self.recipients.values().next() {
                     Some(recipient) => recipient.with(|c| c.name.clone()),
                     None => return Cow::Borrowed("Empty Group"),
                 };