libctf: reimplement many _iter iterators in terms of _next

libctf: reimplement many _iter iterators in terms of _next

Ever since the generator-style _next iterators were introduced, there
have been separate implementations of the functional-style _iter
iterators that do the same thing as _next.

This is annoying and adds more dependencies on the internal guts of the
file format.  Rip them all out and replace them with the corresponding
_next iterators.  Only ctf_archive_raw_iter and ctf_label_iter survive,
the former because there is no access to the raw binary data of archives
via any _next iterator, and the latter because ctf_label_next hasn't
been implemented (because labels are currently not used for anything).

Tested by reverting the change (already applied) that reimplemented
ctf_member_iter in terms of ctf_member_next, then verifying that the
_iter and _next iterators produced the same results for every iterable
entity within a large type archive.

libctf/ChangeLog
2021-03-02  Nick Alcock  <nick.alcock@oracle.com>

	* ctf-types.c (ctf_member_iter): Move 'rc' to an inner scope.
	(ctf_enum_iter): Reimplement in terms of ctf_enum_next.
	(ctf_type_iter): Reimplement in terms of ctf_type_next.
	(ctf_type_iter_all): Likewise.
	(ctf_variable_iter): Reimplement in terms of ctf_variable_next.
	* ctf-archive.c (ctf_archive_iter_internal): Remove.
	(ctf_archive_iter): Reimplement in terms of ctf_archive_next.