Pyhi: A python interpreter

Andrew says:

I've just committed an entirely new Yhc interpreter, written in Python. Pyhi
(as it is known) currently only supports a limited set of the bytecodes
(enough to run Hello World) and is approximately 100 times slower than the C
version.

It uses many of the built in Python features, such as garbage collection, to
attempt to gain some speed and yet is still hopefully written in a fairly
understandable, Pythonic fashion.

Bytecodes are represented as functions which are called using a 'trampoline'
function in order to decouple the Haskell and Python call stacks.

This most definitely a work in progress so don't expect many programs will run
just yet :-)