libctf: Add iteration over non-root types

The existing function ctf_type_iter lets you iterate over root-visible
types (types you can look up by name).  There is no way to iterate over
non-root-visible types, which is troublesome because both the linker
and dumper want to do that.

So add a new function that can do it: the callback it takes accepts
an extra parameter which indicates whether the type is root-visible
or not.

include/
	* ctf-api.h (ctf_type_all_f): New.
	(ctf_type_iter_all): New.

libctf/
	* ctf_types.c (ctf_type_iter_all): New.