Benchmark frame-by-frame JSON serialization again

[jjfiv]

There was a memory leak in the ctypes bindings -- maybe our sense of dumping JSON every frame being too slow is wrong. Also migrate python lib to use ujson -- it's much faster while being API compat to the built-in JSON @etosch

I suspect this is still a non-starter, but it's worth double-checking; can use the new bench-games project.