Gateway: Add connection timeout, add Config to gateway. (#51)

This change fixes tasks hanging due to rare cases of messages being lost between full Discord reconnections by placing a configurable timeout on the `ConnectionInfo` responses. This is a companion fix to [serenity#1255](https://github.com/serenity-rs/serenity/pull/1255). To make this doable, `Config`s are now used by all versions of `Songbird`/`Call`, and relevant functions are  added to simplify setup with configuration. These are now non-exhaustive, correcting an earlier oversight. For future extensibility, this PR moves the return type of `join`/`join_gateway` into a custom future (no longer leaking flume's `RecvFut` type).

Additionally, this fixes the Makefile's feature sets for driver/gateway-only compilation.

This is a breaking change in:
* the return types of `join`/`join_gateway`
* moving `crate::driver::Config` -> `crate::Config`,
* `Config` and `JoinError` becoming `#[non_breaking]`.

This was tested via `cargo make ready`, and by testing `examples/serenity/voice_receive` with various timeout settings.

src/lib.rs

@@ -37,6 +37,7 @@
 //! [`ConnectionInfo`]: struct@ConnectionInfo
 //! [lavalink]: https://github.com/Frederikam/Lavalink
 
+mod config;
 pub mod constants;
 #[cfg(feature = "driver-core")]
 pub mod driver;
@@ -50,6 +51,8 @@ pub(crate) mod info;
 #[cfg(feature = "driver-core")]
 pub mod input;
 #[cfg(feature = "gateway-core")]
+pub mod join;
+#[cfg(feature = "gateway-core")]
 mod manager;
 #[cfg(feature = "serenity")]
 pub mod serenity;
@@ -61,6 +64,7 @@ pub mod tracks;
 mod ws;
 
 #[cfg(feature = "driver-core")]
+/// Opus encoder bitrate settings.
 pub use audiopus::{self as opus, Bitrate};
 #[cfg(feature = "driver-core")]
 pub use discortp as packet;
@@ -86,4 +90,5 @@ pub use crate::{handler::*, manager::*};
 #[cfg(feature = "serenity")]
 pub use crate::serenity::*;
 
+pub use config::Config;
 pub use info::ConnectionInfo;