Impossible to implement a stateful environment - `step` uses an immutable reference to `self`

[benjamincburns]

Per the title, I'm attempting to implement a custom environment for Relearn, but I hit a major roadblock. All methods defined in the Environment trait specify an immutable reference to self. The environment that I'm working on is stateful, and I can't really change this, as the environment wraps a library that I don't control.

Ideally the initial_state would also allow mutation of self, but I can work around that if that's not to your liking.