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`.

examples/serenity/voice_storage/src/main.rs

@@ -27,12 +27,12 @@ use serenity::{
 };
 
 use songbird::{
+    driver::Bitrate,
     input::{
         self,
         cached::{Compressed, Memory},
         Input,
     },
-    Bitrate,
     Call,
     Event,
     EventContext,