Make most things in core opaque to force helper functions and keep API clean

[mlutze]

• GameState refactor: make GameState opaque #89
• Board

[JonathanStarup]

So what should be our local style guide? always use opaque? or always use opaque for pub-involved types?

I think we should make a style doc. for us but also as a collected document of feedback to flix

[mlutze]

Good idea.

I think the rule is that we always use opaque unless the shape of the ADT is actually part of the API. So for Move this is the case, but not for the stuff that we keep changing around.