MiniGrid: Finish the Maze Environments for MiniGrid

[jbloomAus]

A couple of weeks ago I worked with Navpreet to start writing a Maze Environment for Minigrid which is mostly working. Finishing this PR and possibly adding a version that uses Kruskal's algorithm and not Prim's could be a really valuable contribution.

The exciting thing about these environments being part of Minigrid is that it would offer opportunities to study algorithmic distillation and precise, memory-based reasoning in transformers. I think doing this could be the start of a lot of cool work.