First stab at PHP functions calling Python.

At the moment, this is intentionally restrictive: only Python objects wrapped as
W_EmbeddedPyFunc can be called. In other words, with this scheme, Python objects
which define a __call__ method can't be called from PHP. We can loosen this
restriction as and when we work out what we want to do in such cases.
Inline conversion functions.

This is a useful precursors to the *_of_* rename. It also makes the code easier
to read.