[Bug #20868] Fix Method#hash to not change after compaction

	The hash value of a Method must remain constant after a compaction, otherwise
	it may not work as the key in a hash table.

	For example:

	    def a; end

	    # Need this method here because otherwise the iseq may be on the C stack
	    # which would get pinned and not move during compaction
	    def get_hash
	      method(:a).hash
	    end

	    puts get_hash # => 2993401401091578131

	    GC.verify_compaction_references(expand_heap: true, toward: :empty)

	    puts get_hash # => -2162775864511574135