Numba support

[asmeurer]

It's not something I need, but it would be useful to support numba in ndindex.

Numba support means two things:

1. (Optionally) @njit all relevant ndindex methods, so that they can run fast.
2. Register the ndindex types with numba so that they themselves can be used inside of an @njitted function.

I expect both of these would be challenging, but I also imagine this could be very useful for someone who wants to use ndindex in contexts where performance matters.