I like the idea of moving angle stuff into a separate module.  This also paves the way for something else I'd like to think about, which is to generalize `Direction` over the vector space and do a better job distinguishing angles and directions (in 2D we currently conflate the two notions).  (But note I'm not saying we should worry about that for this PR, it's just something I thought I would mention.)

Does `Diagrams.Angle` need to be added to the exported modules list in the `.cabal` file?