Represent Board using Dict
Represent Board using Dict

This change addresses following (invalid) cases with the earlier representation of Board as a List of Cell with Location and Owner.

- Board with a cell owned by both players,
- a cell with no specification of the state,
- or a cell to be both owned and unowned.

vishaltelangre/elm-tic-tac-toe,vishaltelangre/elm-tic-tac-toe