Add `procedure->pointer' to the FFI.

Add `procedure->pointer' to the FFI.

* libguile/foreign.c (make_cif): New procedure, with code formerly in
  `scm_make_foreign_function'.
  (scm_make_foreign_function): Use it.
  (invoke_closure, scm_procedure_to_pointer)[FFI_CLOSURES]: New
  functions.

* libguile/foreign.h (scm_procedure_to_pointer): New declaration.

* module/system/foreign.scm: Export `procedure->pointer' when available.

* test-suite/standalone/test-ffi (f-callback-1, f-callback-2): New
  procedures and related tests.

* test-suite/standalone/test-ffi-lib.c (test_ffi_callback_1,
  test_ffi_callback_2): New functions.

* test-suite/tests/foreign.test ("procedure->pointer"): New test prefix.

* doc/ref/api-foreign.texi (Dynamic FFI): Document `procedure->pointer'.