Add a hint about trace count file names.

doc/user_guide.texi:
	Under "Coverage testing", say how trace count files are named by
	default.  This repeats information from the section "Generating
	trace counts" but saves the user (me) from wondering if coverage
	testing actually worked -- trace count files are prefixed with
	"." by default so a simple "ls" won't show them.
Make some cord functions tail recursive.

Make these functions tail recursive:

	cord.list
	cord.rev_list
	cord.cord_list_to_cord
	cord.cord_list_to_list

library/cord.m:
	As above.
Add test case for cords.

Test the recently changed functions more comprehensively:
cord.list, cord.rev_list, cord.cord_list_to_cord, cord.cord_list_to_list.

The existing test_cord.m is deficient, and did not reveal a bug in the
previous implementation of cord.rev_list (the non-tail-recursive one).

tests/hard_coded/Mmakefile:
tests/hard_coded/test_cord2.exp:
tests/hard_coded/test_cord2.m:
        Add test case.