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").

build.rs

@@ -1,7 +1,4 @@
-#[cfg(all(
-    feature = "driver",
-    not(any(feature = "rustls-marker", feature = "native-marker"))
-))]
+#[cfg(all(feature = "driver", not(any(feature = "rustls", feature = "native"))))]
 compile_error!(
     "You have the `driver` feature enabled: \
     either the `rustls` or `native` feature must be
@@ -12,15 +9,4 @@ compile_error!(
     If you are unsure, go with `rustls`."
 );
 
-#[cfg(all(
-    feature = "twilight",
-    not(any(feature = "zlib-simd", feature = "zlib-stock"))
-))]
-compile_error!(
-    "Twilight requires you to specify a zlib backend: \
-    either the `zlib-simd` or `zlib-stock` feature must be
-    selected.\n\
-    If you are unsure, go with `zlib-stock`."
-);
-
 fn main() {}