Chore: Rework crate features (#139)

All dependencies have been moved to the new "dep:x" and "x?/feature" syntax to remove the bloat from the docs.rs/crates.io/lib.rs feature panes.

Accordingly, this lets us break "rustls" and "native" out from annoying hybrids like "serenity-rustls" or "twilight-native" -- specify your library and your backend, and it should just work.

The complete list of features is now: driver, gateway, serenity, twilight, rustls, native, builtin-queue, simd-json, internals (plus "default" and "full-doc").

.github/workflows/ci.yml

@@ -54,10 +54,10 @@ jobs:
             features: driver rustls
             dont-test: true
           - name: gateway only
-            features: serenity-rustls
+            features: gateway serenity rustls
             dont-test: true
           - name: simd json
-            features: simd-json serenity-rustls driver gateway serenity/simd_json
+            features: simd-json serenity rustls driver gateway serenity?/simd_json
             rustflags: -C target-cpu=native
             dont-test: true