Add a minimal internal WeakMap C-API

This will be used to make the refinement iclasses cache weak, avoiding
a memory leak when creating and using an arbitrary number of
refinements.

internal/gc.h

@@ -188,6 +188,9 @@ RUBY_ATTR_MALLOC void *ruby_mimmalloc(size_t size);
 RUBY_ATTR_MALLOC void *ruby_mimcalloc(size_t num, size_t size);
 void ruby_mimfree(void *ptr);
 void rb_gc_prepare_heap(void);
+VALUE rb_wmap_new_hidden(void);
+VALUE rb_wmap_lookup(VALUE wmap, VALUE key);
+void rb_wmap_aset(VALUE wmap, VALUE key, VALUE val);
 void rb_objspace_set_event_hook(const rb_event_flag_t event);
 VALUE rb_objspace_gc_enable(void *objspace);
 VALUE rb_objspace_gc_disable(void *objspace);

weakmap.c

@@ -504,6 +504,24 @@ wmap_size(VALUE self)
 #endif
 }
 
+VALUE
+rb_wmap_new_hidden(void)
+{
+    return wmap_allocate(0);
+}
+
+VALUE
+rb_wmap_lookup(VALUE self, VALUE key)
+{
+    return wmap_lookup(self, key);
+}
+
+void
+rb_wmap_aset(VALUE self, VALUE key, VALUE val)
+{
+    wmap_aset(self, key, val);
+}
+
 /* ===== WeakKeyMap =====
  *
  * WeakKeyMap contains one ST table which contains a pointer to the object as