Implement the Inspect protocol

[alecho]

When debugging or viewing the game state in logs with inspect it would be easier for humans to see a game board representation as opposed to a struct.

Describe the solution you'd like
Implement the Inspect protocol for %Chex.Game{}

A to_string function would work as well and could be implemented at the same time.

Additional context
https://hexdocs.pm/elixir/1.12/Inspect.html