The behavior of `e` in unchanged, but the type of `fromDirection` has changed.

I was torn between 
- not providing a function `Angle -> R2`, in favor of the new `Direction` semantics
- keeping one around because there's a lot of code, mostly arrowheads, that uses it

Since `e` is a pun on `e^i\theta`, it makes some sense that it takes an Angle.  And it isn't exported from `Diagrams.Preude`, mitigating my concern there.  I'd be fine adding a synonym `fromAngle`.  Should it be exported?

I'm also fine removing `e`, and rewriting code as `unitX & _theta .~ foo` or similar.  Does anyone besides me find that comfortable to read?