Overhaul array conversion.

We now unconditionally convert PHP arrays to Python dicts.

Add an as_list() method to Python dicts which casts a wrapped PHP arrays to lists.

Define a BridgeError exception in PyPy and use it if as_list is called on a non-int-keyed wrapped PHP array dict.

Make wrapped PHP arrays wrap references.

Pass PHP arrays to Python by reference.

Implement some basic mutating ops on wrapped lists/dicts.