Add Unicode decomposition mappings to std::unicode
Add canonical combining class to std::unicode
Add support for performing NFD and NFKD on strings
auto merge of #8445 : Florob/rust/unicode, r=graydon

This adds support for performing Unicode Normalization Forms D and KD on strings.
To enable this the decomposition and canonical combining class properties are added to std::unicode.
On my system this increases libstd's size by ~250KiB.