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/ci.yml

@@ -22,7 +22,7 @@ jobs:
       uses: actions-rs/clippy-check@v1
       with:
         token: ${{ secrets.GITHUB_TOKEN }}
-        args: --all-features
+        args: --features full-doc
 
   test:
     name: Test
@@ -39,6 +39,7 @@ jobs:
           - Windows
           - driver only
           - gateway only
+          - legacy tokio
 
         include:
           - name: beta
@@ -51,8 +52,13 @@ jobs:
             os: windows-latest
           - name: driver only
             features: driver rustls
+            dont-test: true
           - name: gateway only
             features: serenity-rustls
+            dont-test: true
+          - name: legacy tokio
+            features: serenity-rustls-tokio-02 driver-tokio-02
+            dont-test: true
 
     steps:
       - name: Checkout sources
@@ -84,18 +90,18 @@ jobs:
 
       - name: Build all features
         if: matrix.features == ''
-        run: cargo build --all-features
+        run: cargo build --features full-doc
 
       - name: Test all features
         if: matrix.features == ''
-        run: cargo test --all-features
+        run: cargo test --features full-doc
 
       - name: Build some features
         if: matrix.features
         run: cargo build --no-default-features --features "${{ matrix.features }}"
 
       - name: Test some features
-        if: matrix.features
+        if: ${{ !matrix.dont-test && matrix.features }}
         run: cargo test --no-default-features --features "${{ matrix.features }}"
 
   doc:
@@ -131,7 +137,7 @@ jobs:
         env:
           RUSTDOCFLAGS: -D broken_intra_doc_links
         run: |
-          cargo doc --no-deps --all-features
+          cargo doc --no-deps --features full-doc
 
   examples:
     name: Examples