Moved GC.verify_compaction_references to gc.rb

And fixed a segfault by coercion of `Qundef`, when any keyword
argument without `toward:` option is given.

gc.rb

@@ -166,6 +166,24 @@ module GC
   def self.compact
     __builtin_rb_gc_compact
   end
+
+  # call-seq:
+  #    GC.verify_compaction_references(toward: nil, double_heap: nil) -> nil
+  #
+  # Verify compaction reference consistency.
+  #
+  # This method is implementation specific.  During compaction, objects that
+  # were moved are replaced with T_MOVED objects.  No object should have a
+  # reference to a T_MOVED object after compaction.
+  #
+  # This function doubles the heap to ensure room to move all objects,
+  # compacts the heap to make sure everything moves, updates all references,
+  # then performs a full GC.  If any object contains a reference to a T_MOVED
+  # object, that object should be pushed on the mark stack, and will
+  # make a SEGV.
+  def self.verify_compaction_references(toward: nil, double_heap: false)
+    __builtin_gc_verify_compaction_references(toward, double_heap)
+  end
 end
 
 module ObjectSpace