Gateway: Generic Shard and Twilight v0.8 Support (#109)

This PR adds support for twilight v0.8, mainly adapting to significant API changes introduced by v0.7. As a result of these, twilight no longer accepts arbitrary JSON input, so it seemed sensible to adapt our `Shard` design to no longer require the same.

Adding to this, I've added in a trait to allow an arbitrary `Shard` to be installed, given only an implementation of a method to send a `VoiceStateUpdate`. Together, `Sharder::Generic` (songbird::shards::VoiceUpdate) and `Shard::Generic` (songbird::shards::GenericSharder) should allow any library to be hooked in to Songbird.

This PR was tested using `cargo make ready` and by manually testing `examples/twilight`.

examples/twilight/Cargo.toml

@@ -9,10 +9,10 @@ futures = "0.3"
 tracing = "0.1"
 tracing-subscriber = "0.2"
 tokio = { features = ["macros", "rt-multi-thread", "sync"], version = "1" }
-twilight-gateway = "0.6"
-twilight-http = "0.6"
-twilight-model = "0.6"
-twilight-standby = "0.6"
+twilight-gateway = "0.8"
+twilight-http = "0.8"
+twilight-model = "0.8"
+twilight-standby = "0.8"
 
 [dependencies.songbird]
 default-features = false