Library: Add compatibility for legacy Tokio 0.2 (#40)

Adds support to the library for tokio 0.2 backward-compatibility. This should hopefully benefit, and prevent lavalink-rs from being blocked on this feature.

These can be reached using, e.g., `gateway-tokio-02`, `driver-tokio-02`, `serenity-rustls-tokio-02`, and `serenity-native-tokio-02` features.

Naturally, this requires some jiggering about with features and the underlying CI, which has been taken care of. Twilight can't be handled in this way, as their last tokio 0.2 version uses the deprecated Discord Gateway v6.

.github/workflows/docs.yml

@@ -41,7 +41,7 @@ jobs:
         env:
           RUSTDOCFLAGS: -D broken_intra_doc_links
         run: |
-          cargo doc --no-deps --features default,twilight-rustls,builtin-queue,stock-zlib
+          cargo doc --no-deps --features full-doc
 
       - name: Prepare docs
         shell: bash -e -O extglob {0}