Added Transform multiplication and Quaternion::mul_direction

[zesterer]

This adds two new features:

1. A Mul impl for Transform that transforms one in terms of other.

2. A mul_point method on Quaternion, bringing symmetry with Mat4.

[zesterer]

I've been trying to find the time to get round to this, but I'm not currently able to schedule any. I might be able to get this done during the winter break.

[yoanlcq]

No pressure, free time is hard to find, especially during this period.

I must confess I haven't taken any time to dig further, also knowing that it turns out not to be simple, requires tests, and strangely, (given how useful it is) has not been requested by anybody before.

So no problem, progress will come when it wants to!