A few small changes for discussion

[brisvag]

A few changes (easily splittable) for discussion:

1. Use the native Rotation.identity to avoid the hard to parse einops
2. do not unstack/stack Rotations manually if not necessary
3. Use a easier-to-handle (class) way to keep constants. Also, why use POSITION_X when X is just as obvious?
4. I find is_valid names clearer in the code than validate, since I expect a bool from the first, but a validate value from the second