foreign.h presents a more pointer-centric interface

* libguile/foreign.c:
* libguile/foreign.h: Rework interface to be more pointer-centric.
  Details are:
  (SCM_FOREIGN_TYPE_STRUCT, SCM_FOREIGN_TYPE_POINTER): Removed; now the
  pointer in a foreign is first-class. If it points to a native type
  like uint32, then it still has a tag; but if it points to something
  else, like a struct or a pointer or something, then its type is VOID
  (i.e., void*).
  (SCM_FOREIGN_POINTER): Rename from SCM_FOREIGN_OBJECT.
  (SCM_FOREIGN_VALUE_REF, SCM_FOREIGN_VALUE_SET): Rename from
  SCM_FOREIGN_OBJECT_REF and SCM_FOREIGN_OBJECT_SET, to indicate that
  they only work with value types.
  (SCM_FOREIGN_HAS_FINALIZER): Reserve a bit to indicate if the foreign
  pointer in question has a finalizer registered.
  (SCM_FOREIGN_LEN): For void* pointers, optionally store the length in
  bytes of the associated memory region.
  (SCM_FOREIGN_VALUE_P): Rename from SCM_FOREIGN_SIMPLE_P.
  (SCM_VALIDATE_FOREIGN_VALUE): Rename from SCM_VALIDATE_FOREIGN_SIMPLE.
  (scm_take_foreign_pointer): Rename from scm_c_take_foreign. Remove
  scm_c_from_foreign.
  (scm_foreign_type): New accessor.
  (scm_foreign_ref, scm_foreign_set_x): Take some optional args, used
  when dereferencing void pointers.

* libguile/dynl.h:
* libguile/dynl.c (scm_dynamic_pointer): New function, used by
  scm_dynamic_func. Adapt code to foreign.h changes.

* libguile/goops.c (scm_enable_primitive_generic_x)
  (scm_set_primitive_generic_x): Use the SCM_SET_SUBR_GENERIC macro.

* libguile/gsubr.c (create_gsubr): Adapt to API change.
* libguile/gsubr.h (SCM_SUBRF, SCM_SUBR_GENERIC): Store the pointer
  directly, not indirected.

* libguile/snarf.h (SCM_DEFINE, SCM_IMMUTABLE_FOREIGN): Store subr
  pointers directly. Adapt to SCM_FOREIGN_TYPE_VOID change.

* libguile/vm-i-system.c (subr-call): Access the void* directly.

libguile/vm-i-system.c

@@ -842,7 +842,7 @@ VM_DEFINE_INSTRUCTION (80, subr_call, "subr-call", 1, -1, -1)
   nargs = FETCH ();
   POP (foreign);
 
-  subr = SCM_FOREIGN_OBJECT_REF (foreign, void*);
+  subr = SCM_FOREIGN_POINTER (foreign, void);
 
   VM_HANDLE_INTERRUPTS;
   SYNC_REGISTER ();