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`.

Cargo.toml

@@ -13,6 +13,7 @@ repository = "https://github.com/serenity-rs/songbird.git"
 version = "0.2.2"
 
 [dependencies]
+derivative = "2"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 tracing = { version = "0.1", features = ["log"] }
@@ -104,12 +105,12 @@ default-features = false
 
 [dependencies.twilight-gateway]
 optional = true
-version = ">=0.5, <0.7"
+version = "0.8"
 default-features = false
 
 [dependencies.twilight-model]
 optional = true
-version = ">=0.5, <0.7"
+version = "0.8"
 default-features = false
 
 [dependencies.typemap_rev]