Restricting phantom types

[kvark]

It feels a little bit weird that we can put any type in place of the second parameter of LeftQuaternion or EulerAngles. Perhaps, this is not a problem on mint side, since you can always abuse it if you really want to.

[Ralith]

A (private?) trait might make the code clearer. On the other hand, mint types should most often be seen in concrete form in other interfaces, where this is not a concern.