basic/hashmap,set: inline trivial set_iterate() wrapper

The compiler would do this to, esp. with LTO, but we can short-circuit the
whole process and make everything a bit simpler by avoiding the separate
definition.

(It would be nice to do the same for _set_new(), _set_ensure_allocated()
and other similar functions which are one-line trivial wrappers too. Unfortunately
that would require enum HashmapType to be made public, which we don't want
to do.)