Remove Matrix trait impls for RotationMat

These operations could potentially mess up the orthoganality of the rotation matrix. We could have made these fail if an the resulting matrices were not orthogonal, but it is preferable to enforce this at compile time rather than run time. More operations should be added in the future with this restriction in mind.
Move rotation module into transform module

This decouples these types from the more general math types. It also makes more sense, because rotations are indeed a type of transformation.