Support linking to Pyston rather than CPython

I'm not putting any pressure on supporting Pyston [or any other JIT Python], just think of this as a FYI on the interesting thing I discovered; the read on e.g. ref counting vs. former full GC.

[It seems Numba is supported now: https://github.com/stevengj/PyCall.jl/issues/220

I wander if other than Numba JIT, such as Pyston, or more than one JIT Python implementation, is a good idea to support.]

https://blog.pyston.org/2016/05/25/pyston-0-5-released/
"Pyston is now using reference counting.
[..]
We used a GC initially to try to get more performance. But applying a tracing GC to a refcounting C API, such as the one that Python has, is risky and comes with many performance pitfalls. And most challengingly, Pyston wants to support the large amount of code that has been written that relies on the special properties that refcounting provides (predictable immediate destruction). We found that we had to go to greater and greater lengths to support these programs
[..]
We are very happy to announce: we can run NumPy, unmodified."