add simple foreign finalization, and pointer support

add simple foreign finalization, and pointer support

* libguile/foreign.h:
* libguile/foreign.c (scm_foreign_set_finalizer_x): New function, for a
  limited form of finalization (like `free').
  (scm_alignof, scm_sizeof, parse_ffi_type, fill_ffi_type): For the
  purposes of make-foreign-function, treat '* (the asterisk symbol) as a
  pointer.

* module/system/foreign.scm: Export foreign-set-finalizer!.