Move foreign pointers off scm_words

* libguile/foreign.h: Give pointers a struct type.  Adapt to use it
internally.
* libguile/foreign.c: Allocate pointers with scm_allocate_tagged.

libguile/foreign.h

@@ -52,11 +52,59 @@ typedef enum scm_t_foreign_type scm_t_foreign_type;
 
 typedef void (*scm_t_pointer_finalizer) (void *);
 
-#define SCM_POINTER_P(x) (SCM_HAS_TYP7 (x, scm_tc7_pointer))
+struct scm_pointer
+{
+  scm_t_bits tag_and_size;
+  uintptr_t address;
+
+  /* Pointers can be allocated with a number of extra tail words.  These
+     are useful when you have a pointer derived from an object; you need
+     the object to stay alive as long as the derived pointer is alive.
+     Storing the object in a slot of the derived pointer will allow for
+     this.  */
+  SCM gc_objects[];
+};
+
+static inline int
+scm_is_pointer (SCM x)
+{
+  return SCM_HAS_TYP7 (x, scm_tc7_pointer);
+}
+
+// Most of these SCM-to-C-struct / C-struct-to-SCM functions are named
+// "scm_to_foo" / "scm_from_foo", but here in the FFI we run afoul of
+// the already-existing, already-well-named scm_to_pointer and
+// scm_from_pointer.  Let's just prefix with "scm_i_" instead.
+static inline struct scm_pointer *
+scm_i_to_pointer (SCM x)
+{
+  if (!scm_is_pointer (x))
+    abort ();
+  return (struct scm_pointer *) SCM_UNPACK_POINTER (x);
+}
+
+static inline SCM
+scm_i_from_pointer (struct scm_pointer *x)
+{
+  return SCM_PACK_POINTER (x);
+}
+
+static inline void*
+scm_pointer_value (struct scm_pointer *p)
+{
+  return (void *) p->address;
+}
+
+static inline size_t
+scm_pointer_gc_object_count (struct scm_pointer *p)
+{
+  return p->tag_and_size >> 16;
+}
+
+#define SCM_POINTER_P(x) (scm_is_pointer (x))
 #define SCM_VALIDATE_POINTER(pos, x)		\
   SCM_MAKE_VALIDATE (pos, x, POINTER_P)
-#define SCM_POINTER_VALUE(x)			\
-  ((void *) SCM_CELL_WORD_1 (x))
+#define SCM_POINTER_VALUE(x) (scm_pointer_value (scm_i_to_pointer (x)))
 
 SCM_API void *scm_to_pointer (SCM pointer);
 SCM_API SCM scm_from_pointer (void *, scm_t_pointer_finalizer);