[bindings] Don't export new functions with unexportable types

[bindings] Don't export new functions with unexportable types

`CommitmentTransaction::new_with_auxiliary_htlc_data()` includes a
unbounded generic parameter which we can't concretize and it's of
limited immediate use for users in any case. We should eventually
add a non-generic version which uses `()` for the generic but that
can come later.

`CommitmentTransaction::htlcs()` returns a reference to a Vec,
which we cannot currently map. It should, however, be exposed to
users, so in the future we'll need to have a duplication function
which returns Vec of references or a cloned Vec.