…erenity-rs#6)

* Depend on Serenity's `next` branch on Songbird's `next` branch

* Update the examples to use the `next` branch

Serenity's cache design has changed, this (finally) prevents the examples from failing to compile on CI.

Adds generics to any `Id` types on `Call`. Also includes the overlooked `Songbird::get_or_insert`.

Closes serenity-rs#94. Tested using `cargo make ready`.

This matches a recent serenity change where `ClientBuilder` no longer has an explicit lifetime parameter.

This was tested using `cargo make ready`.

	modified:   src/shards.rs

	modified:   src/handler.rs

	modified:   src/handler.rs
	modified:   src/shards.rs

	modified:   ../../../Cargo.toml
	modified:   ../../../src/driver/mod.rs
	modified:   ../../../src/input/dca.rs
	modified:   ../../../src/input/error.rs
	modified:   ../../../src/input/ffmpeg_src.rs
	modified:   ../../../src/input/metadata.rs
	modified:   ../../../src/input/ytdl_src.rs
	modified:   ../../../src/ws.rs

	modified:   Cargo.toml
	modified:   src/driver/mod.rs
	modified:   src/input/dca.rs
	modified:   src/input/error.rs
	modified:   src/input/ffmpeg_src.rs
	modified:   src/input/metadata.rs
	modified:   src/input/ytdl_src.rs
	modified:   src/ws.rs

	modified:   src/driver/mod.rs
	modified:   src/input/dca.rs
	modified:   src/input/ffmpeg_src.rs
	modified:   src/input/metadata.rs
	modified:   src/input/ytdl_src.rs
	modified:   src/tracks/error.rs
	modified:   src/ws.rs

	modified:   Cargo.toml
	modified:   examples/serenity/voice_receive/Cargo.toml
	modified:   src/input/metadata.rs
	modified:   src/ws.rs

	modified:   examples/serenity/voice_receive/Cargo.toml

	modified:   README.md

	modified:   .github/workflows/ci.yml

	modified:   .github/workflows/ci.yml

	new file:   .cargo/config.toml