Implement low-level python bindings for `pcg`

[lockshaw]

Implement low-level Python bindings around pcg's ffi. This should include common utilities for handling error reporting, validation, logging, etc. These will likely be used a very large number of times in the course of implementing the ffi (as these behaviors are shared across all ffi functions), so they should be well-developed and easy/concise to use and update. See (somewhat outdated) examples in bindings/python/python/flexflow/pcg/raw.py