* add `set_state`, `get_state` methods to save/restore environment state
* new flags: `use_backgrounds`, `restrict_themes`, `use_monocrhome_assets`
* switch to use `gym3` instead of `libenv` + `Scalarize`, `gym` and `baselines.VecEnv` interfaces are still available with the same names, the `gym3` environment is called `ProcgenGym3Env`
* zero initialize more member variables
* changed `info` dict to have more clear keys, `prev_level_complete` tells you if the level was complete on the previous timestep, since the `info` dict corresponds to the current timestep, and the current timestep is never on a complete level due to automatic resetting. Similarly, `prev_level_seed` is the level seed from the previous timestep.
* environment creation should be slightly faster