cl_assert_equal_wcs(n): wide character comparisons

Allow comparisons between wide strings (`wchar_t *`) using `cl_assert_equal_wcs`.

I admit that I haven't done an exhaustive investigation to see what platforms might *not* have `wchar.h`.  This has been in POSIX since 2003, and in the C standard since C89(!), so one would *hope* that approximately everything would have it, but I may be horribly mistaken.