Codepoints and noncharacters.

- Added basic defs for codepoints, such as types and a few metrics.
- Added mech to check if a code point is noncharacter.
- Added unit test to exhaustively test all code points for noncharacter and compare the results to IsNoncharacter.
Started on encoding traits.
First pass implementation of utf8.
Encode and Decode now use iterators rather than raw pointers.