move foreign function interface to its own module

move foreign function interface to its own module

* libguile/foreign.h:
* libguile/init.c: Change so that init just registers an extension,
  later called by foreign.scm.

* libguile/foreign.c (scm_init_foreign): Define constants for the
  various foreign types.

* module/Makefile.am:
* module/system/foreign.scm: New module, for the foreign function
  interface.