npbackend: Refactored folder structure.
npbackend: Refactored folder structure.

bhpy -> npbackend, to reflect the naming described in the paper.
bhpy/backend -> npbackend/target, to reflect the backend "target".

Instead of the dual "npbackend backend", it is now the "npbackend target".
We can then more easily explain that backend, targets something.

ENV-VAR BHPY_TARGET has been added but for compatibility BHPY_BACKEND is
still there.

bh107/benchpress,bh107/benchpress,bh107/benchpress,bh107/benchpress