ponyquad/hot reload: Less locking

[HoneyPony]

The implementation of poni_hot_lookup in Rust uses a Mutex for a global variable.

This is mostly fine, but it is less efficient than necessary to lock the lock for every single global. Ideally, the poni_hot_lookup API would be updated to take the mutex/opaque context as an argument, and then we would only have to lock the lock once (assuming also the existence of poni_hot_lookup_take_context() and poni_hot_lookup_finish(void *context) or something).