Added named constant for 2-unit utf16 code point encoding offset.
Started on transcoding.

I have a few ideas kicking around about fast vs safe transcoding.
For now, I'm bashing these in favor of safe, straightforward transcoding.
We'll revisit this later.
unchecked_encoder_c is now parameterized on an encoding directly.
Unchecked encoder will now encode code points >= 0x110000.
Implemented transcoder.
Renamed encoding/dynamic.* encoding/dynamic_encoding.*.
Added rough dynamic transcoder.