Add ExactSize and DoubleEnded impls to GridRow and Col Iter

[luca3s]

I wrote implementations of ExactSizeIterator and DoubleEndedIterator for GridRowIter and GridColIter.

• adds one usize of additional memory usage to both types
• makes both more versatile and maybe even faster (e.g. when collecting into a vec ExactSizeIterator should reduce the needed allocations)

The changes to Iterator are bigger than actually needed, but now the next and next_back functions look very similar.
Iterator still passes all test. I also added tests for all new code.

Maybe closes #38, which i just notices while writing this.