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").

src/lib.rs

@@ -2,6 +2,7 @@
     html_logo_url = "https://raw.githubusercontent.com/serenity-rs/songbird/current/songbird.png",
     html_favicon_url = "https://raw.githubusercontent.com/serenity-rs/songbird/current/songbird-ico.png"
 )]
+#![cfg_attr(docsrs, feature(doc_auto_cfg))]
 #![deny(missing_docs)]
 #![deny(rustdoc::broken_intra_doc_links)]
 //! ![project logo][logo]
@@ -9,7 +10,7 @@
 //! Songbird is an async, cross-library compatible voice system for Discord, written in Rust.
 //! The library offers:
 //!  * A standalone gateway frontend compatible with [serenity] and [twilight] using the
-//!  `"gateway"` and `"[serenity/twilight]-[rustls/native]"` features. You can even run
+//!  `"gateway"` and `"[serenity/twilight]"` plus `"[rustls/native]"` features. You can even run
 //!  driverless, to help manage your [lavalink] sessions.
 //!  * A standalone driver for voice calls, via the `"driver"` feature. If you can create
 //!  a [`ConnectionInfo`] using any other gateway, or language for your bot, then you

src/serenity.rs

@@ -1,5 +1,5 @@
 //! Compatability and convenience methods for working with [serenity].
-//! Requires the `"serenity-rustls"` or `"serenity-native"` features.
+//! Requires the `"serenity"` feature.
 //!
 //! [serenity]: https://crates.io/crates/serenity/0.9.0-rc.2