Improve Angle

[T0mstone]

This is a suggestion for a slight improvement to the Angle type.

1. It replaces the usage of 2. * PI with TAU: This constant exists, so why not use it?
2. It adds two new units that an angle can be converted to/from. I think gon is used in engineering, so it could prove useful; Revolutions is just good to have in any case (say you want to rotate something 1/3 of a turn, now it's super easy)
3. It removes the multiplication of two angles. Multiplying two angles is mathematically only useful if talking about solid angles, which this definetely wasn't. I don't see a context in which you would multiply an angle by another angle. Rather, you'll probably want to multiply an angle by a simple factor (something like "2 times that angle" or "1/3 of that angle"), so I implemented that. Dividing an angle by an angle is well-defined, so I kept it.

I don't think the changed API was used anywhere yet, but I'm not familiar with the codebase. This just jumped out at me when reading the PR that introduces angles, so I thought to just try a PR of my own.

[hannobraun]

Thank you for the pull request, @T0mstone, these are some great changes!