Change DataType from enum to class
Break DataType enums into QL (default) and SQL types
Make data type conversion pluggable so that new types can be introduced

As part of the process:
- static type conversion in QL package (such as Literal) has been
removed
- several utility classes have been broken into base (QL) and extended
(SQL) parts based on type awareness
- operators (+,-,/,*) are
- due to extensibility, serialization of arithmetic operation has been
slightly changed and pushed down to the operator executor itself