Investigate switching to heap allocated board in the myopic-board crate API

[th0masb]

At the moment there are several places in the myopic-board crate where we return an explicit implementation type allocated on the stack MutBoardImpl. It makes more sense from a SWE perspective to return the generic trait type MutBoard which forces returning generic a fat pointer to the heap allocated object Box<dyn Mutboard>. All our generic methods defined elsewhere should work and we can probably remove the awkward parameterization on the EvalBoard trait.