Try rewriting `pandad` in Python

This allows us to remove the parallel C++ Panda interfacing code we maintain just for `pandad`.

The main challenge here is maintaining the timings and performance of the `can` and `sendcan` loops, but it shouldn't be too bad.

As a side benefit, this will also help a bit with boot time, since we currently connect to the pandas twice (pandad.py and pandad.cc).