[Patch] Fix libstdc++/13341 (speed-up <wchar_t>::do_narrow/widen)

[Benjamin Kosnik]

>Updated patch below, tested, as usual, both gnu/generic on x86-linux.

Cool.

What about Jerry's two points (256 vs. 128 table, and the testsuite for
narrow)?  FWIW, ctype<char> has a 256 element table. Seems like the
right approach.

-benjamin