libctf: implementation definitions related to file creation

libctf: implementation definitions related to file creation

We now enter a series of commits that are sufficiently tangled that
avoiding forward definitions is almost impossible: no attempt is made to
make individual commits compilable (which is why the build system does
not reference any of them yet): the only important thing is that they
should form something like conceptual groups.

But first, some definitions, including the core ctf_file_t itself.  Uses
of these definitions will be introduced in later commits.

libctf/
	* ctf-impl.h: New definitions and declarations for type creation
	and lookup.