Make char::to_upper and char::to_lower support Unicode

[catamorphism]

currently they're ASCII-only.

[graydon]

Unlikely. I think this is over the boundary of "stuff we're going to punt to libicu and provide in std, not core".

If it's confusing to have them in char, remove them from char and put them in libc, taking a c_char, to clarify that they do not work on Full Unicode. The unicode logic in core::char is confirmant, but minimal, not intended to be complete.

[graydon]

Also dupe of #1347