Avoid warning from C compiler about mismatched pointer types.

Avoid warning from C compiler about mismatched pointer types
when the --optimise-constructor-last-call optimisation is enabled
in high-level C grades.

compiler/ml_unify_gen.m:
	Cast the pointer of a cell field to the type of the address
	variable that will hold it.
Fix the documentation of map.union/4.

library/map.m:
	Add a missing word.

	Refer to the higher-order argument as a closure rather than
	a predicate since the description covers both the function
	and predicate versions of map.union.