Wrap op_class() method.
Better result handling.

- default result handling for root op
  - unary/binary ops set and push target
  - everything else creates a new SV and pushes it
- when the root op returns no value, it's assumed
  to have handled the output itself (ex. returning a list)
- fixed handling of += (checks OPf_STACKED, not sure whether it
  should be a separate add_assign optype or an is_assigning flag).
Add basic test infrastructure.
Add basic addition tests.
Move old test file to old_code.