Change fillByte to an Evaluatable
- Support fill bytes that use a character and runtime value encoding.
- Move fill byte out of the RuntimeData. Only two unparsers really need
this (hexbinary and string unparsers), so this saves a little space in
RuntimeData.
- Add tests to ensure fillByte the error cases are checked (e.g.
multi-byte character, non-byte size characters, fill byte with more
than one charcter, etc.)
