libctf: dedup: chase API changes: use the public API more

libctf: dedup: chase API changes: use the public API more

To get ready for the deduplicator changes, we chase the API changes to
things like ctf_member_next, and add support for prefix types (using the
suffix where appropriate, etc).  We use the ctf-types API for things like
forward lookup, using the private _tp functions to reduce overhead while
centralizing knowledge of things like the encoding of enum forwards outside
the deduplicator.

No functional changes yet.