Driver: Move Bitrate import out of crate root. (#53)

This is a simple organisational change which moves `crate::Bitrate` to `crate::driver::Bitrate` to slightly clean up the crate root.

This has been tested using `cargo make ready`.

src/driver/mod.rs

@@ -32,7 +32,8 @@ use crate::{
     Event,
     EventHandler,
 };
-use audiopus::Bitrate;
+/// Opus encoder bitrate settings.
+pub use audiopus::{self as opus, Bitrate};
 use core::{
     future::Future,
     pin::Pin,