Extended unicode support

by Dmitry Olshansky for Digital Mars

Make complex Unicode stuff a piece of cake in the D programming language. Major points are: normalization; faster and always up to date implementation of common isXXX character properties; a proper case insensitive comparators for strings; grapheme clusters. Possible extras: support for legacy encodings, generic mutli-level lookup tables, i.e. Trie for std.container.