Couple &str-boards

[aritmos]

Within the current implementation, the &str-boards within solvers::utils::boards are decoupled. i.e. nothing links a boards unsolved state from its solved state (beyond the const's name).

Ideally as testing becomes automated both the unsolved board and the solved board are coupled within a wrapper type. This not only makes the testing more ergonomic but ensures the mutual existance of each board state, which should always be the case.