marshal.c: Support dump and load of a Hash with the ruby2_keywords
	 flag

	It is useful for a program that dumps and load arguments (like drb).
	In future, they should deal with both positional arguments and keyword
	ones explicitly, but until ruby2_keywords is deprecated, it is good to
	support the flag in marshal.

	The implementation is similar to String's encoding; it is dumped as a
	hidden instance variable.

	[Feature #16501]