Separate the async bluetooth handling from networking code.

They're not at all related, besides both being asynchronous. This change adds
a little extra code in response_async(), but makes this code more readable
and reduces the unnecessary indirection.

This change also makes the build system slightly more parallel, by dropping
the dependency on bluetooth_traits from net_traits.

components/bluetooth_traits/lib.rs

@@ -108,7 +108,3 @@ pub enum BluetoothResponse {
     EnableNotification(()),
     WatchAdvertisements(()),
 }
-
-pub trait BluetoothResponseListener {
-    fn response(&mut self, response: BluetoothResponseResult);
-}