libctf: add hash traversal helpers

libctf: add hash traversal helpers

There are two, ctf_dynhash_iter and ctf_dynhash_iter_remove: the latter
lets you return a nonzero value to remove the element being iterated
over.

Used in the next commit.

libctf/
	* ctf-impl.h (ctf_hash_iter_f): New.
	(ctf_dynhash_iter): New declaration.
	(ctf_dynhash_iter_remove): New declaration.
	* ctf-hash.c (ctf_dynhash_iter): Define.
	(ctf_dynhash_iter_remove): Likewise.
	(ctf_hashtab_traverse): New.
	(ctf_hashtab_traverse_remove): Likewise.
	(struct ctf_traverse_cb_arg): Likewise.
	(struct ctf_traverse_remove_cb_arg): Likewise.