"char": make "to_lower"/"to_upper" work for non-ASCII characters

[kud1ing]

This probably should be implemented by means of the non-exported unicode.rs in libcore.

[kud1ing]

See "case mapping" on http://www.unicode.org/faq/casemap_charprop.html

[kud1ing]

See #5822

[bstrie]

Nominating for Maturity 5, Production Ready

[graydon]

accepted for backwards-compatible milestone

[graydon]

more generally, figure out what to do with "full unicode" in libstd

[thestinger]

These functions no longer exist, so this isn't a backwards compatibility issue.

[catamorphism]

Nominating for milestone 5, production-ready

[thestinger]

Replaced with #9084.