std::unicode: remove unused category tables
Implement lower, upper case conversion for char
Remove code duplication

Remove whitespace

Update documentation for to_uppercase, to_lowercase
auto merge of #12561 : pzol/rust/char-case, r=alexcrichton

Added common and simple case folding, i.e. mapping one to one character mapping. For more information see http://www.unicode.org/faq/casemap_charprop.html

Removed auto-generated dead code which wasn't used.