libctf: add ctf_forwardable_kind

libctf: add ctf_forwardable_kind

The internals of the deduplicator want to know if something is a type
that can have a forward to it fairly often, often enough that inlining
it brings a noticeable performance gain.  Convert the one place in
libctf that can already benefit, even though it doesn't bring any sort
of performance gain there.

libctf/
	* ctf-inlines.h (ctf_forwardable_kind): New.
	* ctf-create.c (ctf_add_forward): Use it.