WIP: Numba call

I've addressed comments here

https://github.com/inducer/loopy/pull/89#pullrequestreview-40091981

except for the part about generating an intermediate function from the get_executor method. I read the source on the PyOpenCL executor, and there it makes sense to have an intermediary to handle queue, but Numba's jit function is immediately usuable.

If you can explain a little more what you had in mind, I'm happy to code it up.

I also have further additions towards tagging an iname w/ g.0 and having it generate a numba.prange, but that's a larger change maybe better in a separate PR.