[ocaml-ctypes] Removing ctypes.foreign.{threaded,unthreaded}

[Jeremy Yallop]

Current releases of ctypes include two implementations of the Foreign
module, available in findlib subpackages:

   - ctypes.foreign.threaded
   - ctypes.foreign.unthreaded

The appropriate implementation is selected automatically, depending on
whether threads are in use.

I'd like to remove these subpackages, replacing them with a single
implementation of Foreign which always depends on the threads library.

If this change is likely to cause problems for you, please speak up,
either here or on the GitHub PR:

    https://github.com/ocamllabs/ocaml-ctypes/pull/419