Simplify Config structure

This simplifies some of the `Config` structure, in particular this
means:

Parameters which are meaningfully equivalent longer stored in
an `Option<T>`, an example of this is `channels`. If you don't
want to join any channels you simply leave it as empty instead.
In effect, `None` is behaviorally equivalent to `vec![]`.

We don't allocate when accessing certain configuration options.
For example, when accessing `channels` we used to allocate a
vector to handle the "empty case", we simply return the slice
corresponding to the list of channels instead.

We skip serializing empty or optional configuration fields.
From a deserialization perspective this is already something
that was mostly supported through use of `Option<T>` and
`#[serde(default)]`.

examples/build-bot.rs

@@ -15,7 +15,7 @@ async fn main() -> irc::error::Result<()> {
     let config = Config {
         nickname: Some("irc-crate-ci".to_owned()),
         server: Some("irc.pdgn.co".to_owned()),
-        use_ssl: Some(true),
+        use_ssl: true,
         ..Default::default()
     };