Driver: Automate (re)connection logic (#81)

This PR adds several enhancements to Driver connection logic:
* Driver (re)connection attempts now have a default timeout of around 10s.
* The driver will now attempt to retry full connection attempts using a user-provided strategy: currently, this defaults to 5 attempts under an exponential backoff strategy.
* The driver will now fire `DriverDisconnect` events at the end of any session -- this unifies (re)connection failure events with session expiry as seen in #76, which should provide users with enough detail to know *which* voice channel to reconnect to. Users still need to be careful to read the session/channel IDs to ensure that they aren't overwriting another join.

This has been tested using `cargo make ready`, and by setting low timeouts to force failures in the voice receive example (with some additional error handlers).

Closes #68.

src/events/core.rs

@@ -33,12 +33,22 @@ pub enum CoreEvent {
     DriverConnect,
     /// Fires when this driver successfully reconnects after a network error.
     DriverReconnect,
+    #[deprecated(
+        since = "0.2.0",
+        note = "Please use the DriverDisconnect event instead."
+    )]
     /// Fires when this driver fails to connect to a voice channel.
     DriverConnectFailed,
+    #[deprecated(
+        since = "0.2.0",
+        note = "Please use the DriverDisconnect event instead."
+    )]
     /// Fires when this driver fails to reconnect to a voice channel after a network error.
     ///
     /// Users will need to manually reconnect on receipt of this error.
     DriverReconnectFailed,
+    /// Fires when this driver fails to connect to, or drops from, a voice channel.
+    DriverDisconnect,
     /// Fires whenever the driver is assigned a new [RTP SSRC] by the voice server.
     ///
     /// This typically fires alongside a [DriverConnect], or a full [DriverReconnect].