move to `cref` FFI.
move to `cref` FFI.

Somewhat simpler than the bytecode backend.
Note: need to settle on a standard type notation for integer types,
  and this needs to be pushed all the way into ffi/gen.py.

samrushing/irken-compiler,samrushing/irken-compiler,samrushing/irken-compiler,samrushing/irken-compiler