Roadmap to 2.0.0

[yoriyuki]

• Split Camomile into base, ua (Unicode Algorithm), charencodings and locales
• Deprecate mutable Unicode strings
• Integrate stdlib's Uchar? (need to modify internal, may better to be postponed)
• Better error reporting? (avoid using exception etc.)

Support for Unicode 10 or 11 would be better to be postponed.

[yoriyuki]

I will start working on splitting Camomile. @rgrinberg