Factor Response into apdu module; improved debugging

This commit merges the `apdu` and `response` modules: the responses are
APDU responses, and so the two are related.

This also moves the `trace` logging into the APDU type, which allows it
to display `Debug` output for APDUs and responses, which makes it easier
to understand what's going on (and will be even better once instructions
are converted into an enum so you can actually see what's happening).

src/lib.rs

@@ -155,7 +155,6 @@ mod metadata;
 pub mod mgm;
 #[cfg(feature = "untested")]
 pub mod msroots;
-mod response;
 #[cfg(feature = "untested")]
 mod serialization;
 #[cfg(feature = "untested")]