Add some more standard library procedures.

library/bimap.m:
	Add a predicate for testing semantic equality of bimaps.

library/assoc_list.m:
	Add fold_values with two or three accumulator arguments.

library/int.m:
	Add fold_{up,down} with three accumulator arguments.

library/map.m:
library/tree234.m;
	Add foldl_values with two or three accumulator arguments.

NEWS:
	Announce the above additions.

	Reformat the existing changes to the pqueue module slightly