Add the predicate maybe.fold2_maybe/6 to the standard library.

Add a predicate version of ground_pseudo_type_desc_to_type_desc.

library/maybe.m:
	Add the above predicate.

	Re-order some existing declarations as per the coding standard.

library/type_desc.m:
	Add a predicate verison of ground_pseudo_type_desc_to_type_desc.

NEWS:
	Announce the above.