provide mutable iterators

[rursprung]

thanks for this useful library! as noted in other issues it's great to reflect game boards 🙂

the iterator methods are really handy, however i lack the possibility to have a mutable iterator. i briefly tried adding this myself but ran into lifetime issues with self, so i'm skipping it for now (i instead just iterate over the indices and use get_mut on them), but thought i'd raise the issue to put this out there in case someone else would (a) know how to implement this and (b) also need it. feel free to close the issue if you don't think that this is worth it / doable as i have a working solution.