Add support for unicode case folding

Currently, core includes no unicode case folding functions.

Full locale-dependent support as explained in http://www.w3.org/International/wiki/Case_folding is probably out of scope for the core library, however, one following a default folding as mentioned in http://www.unicode.org/faq/casemap_charprop.html#5 should probably be included to make working with strings easier.
