Return Result<Signature> instead of modifying args in ChannelKeys

Return Result<Signature> instead of modifying args in ChannelKeys

This cleans up sign_local_commitment somewhat by returning a
Result<Signaure, ()> over the local commitment transaction instead
of modifying the struct which was passed in.

This is the first step in making LocalCommitmentTransaction a
completely pub struct, using it just to communicate enough
information to the user to allow them to construct a signaure
instead of having it contain a bunch of logic.

This should make it much easier to implement a custom ChannelKeys
by disconnecting the local commitment transaction signing from our
own datastructures.