Add char, unit, int32, int64 to built-in comparators and update ppx_deriving_popper

[bikallem]

Fixes #47.

Summary:

1. Add char, unit, int32, int64 and unit to built-in comparators
2. Add bytes and Bytes module to Sample module.
3. Updates ppx_deriving_popper to generate Sample.t code for char, unit and int64
4. Update tests to exercise new functionality