Provide a lower level unsafe board

[LimeEng]

It might make sense to provide a lower level board that guarantees nothing, for higher performance. This lower level board would simply assume that all inputs are valid. The higher level board would still provide error checking.

Naming suggestions:

• High level: othello::Game
  Low level: othello::Board

• High level: othello::Board
  Low level: othello::UnsafeBoard

  The high level board could additionally keep track of whose turn it is, if someone passed their turn and so on.

[LimeEng]

This might not be as interesting from a performance perspective anymore since #51 landed, as only place_stone have obvious error-checking that can be removed.