feat: v8 encryption modes (#264)

This PR adds support for the new AEAD cryptosystems advertised by Discord, AES256-GCM and XChaCha20Poly1305. These schemes will shortly become mandatory, and provider stronger integrity/authentication guarantees over the cleartext portions of any voice packet by correctly specifying additional authenticated data.

To provide smooth switchover, we've added basic negotiation over the `CryptoMode`. This ensures that any clients who are manually specifying one of the legacy modes will automatically migrate to `Aes256Gcm` when Discord cease to advertise their original preference.

Closes #246.

---------

Co-authored-by: Kyle Simpson <kyleandrew.simpson@gmail.com>

src/input/sources/mod.rs

@@ -38,13 +38,16 @@ impl AsyncReadOnlySource {
     }
 
     /// Gets a reference to the underlying reader.
+    #[allow(clippy::borrowed_box)] // FIXME: Changing this may break compatibility. remedy in v0.5
+    #[must_use]
     pub fn get_ref(&self) -> &Box<dyn AsyncRead + Send + Sync + Unpin> {
         &self.stream
     }
 
     /// Unwraps this `AsyncReadOnlySource`, returning the underlying reader.
+    #[must_use]
     pub fn into_inner<R>(self) -> Box<dyn AsyncRead + Send + Sync + Unpin> {
-        self.stream.into()
+        self.stream
     }
 }