Add first-class python backend

[jessegrabowski]

We've talked about this before so I just want an issue to track it. The idea is that:

• perform should be in python, but it should be as simple as possible
• mode = 'py' should have python-specific dispatches where we can put mode full-featured "performance" python code. An example would be the QZ Op. All that should be moved out of perform, perform should just do scipy.linalg.qz
• Replace the default FAST_COMPILE with python mode (no compile is really fast right?)

There might not actually be much to do to get this off the ground, just some refactor to make things nice and hide complexity in certain cases. Not sure about how this would impact random variables, scan, etc.

[ricardoV94]

Note: python mode shouldn't use composite / fusion! That is usually orders of magnitude worse than regular numpy