Should boundary(::Rectangle) use simpler types?

[dlfivefifty]

I'm wondering if it would be more natural to have a notion of a rectangle embedded in a higher dimensional space rather than representing them as affine mapped rectangles... eg I would have expected the boundary of a 2D rectangle to be a union of LineSegments instead of affine mapped unit intervals.

I guess the two definitions are equivalent so maybe its just a question of making a type alias with prettier printing? That is, we could have LineSegment(a,b) create an affine mapped interval.

[daanhb]

Sure, we could have an alias. It would be good to have more general faces and edges etcetera, but of course multiple other packages provide that. I'd like to get #141 out of the way so that we can move on, I have some code depending on it in the meantime so I'll merge and fix any issues that arise.

[daanhb]

Dup of #100