Add io.{write,print}_line to the standard library.

Add the predicates io.write_line and io.print_line to the standard library.
These are versions of io.write and io.print respectively, that in addition to
printing their argument also print a final newline.  While this is no different
to calling io.{write,print} and then calling io.nl, it is a little more concise
which can be convenient in some circumstances.

library/io.m:
	Add the above predicates.

compiler/error_util.m:
	Avoid ambiguity with the write_line/3 predicate defined
	in this module.

NEWS:
	Announce the new predicates.

tests/hard_coded/write.m:
	Use up-to-date syntax in this module.

	Call write_line in a few spots.