Breaking cleanup

[ffreyer]

Extracts some breaking changes from #238:

• replace Rect{N, T}() = Rect(Vec{N, T}(Inf), Vec{N, T}(-Inf)) with Rect(Vec{N, T}(NaN), Vec{N, T}(0)) and let Integers error
• change self_intersections() to return a vector of tuples corresponding to the intersecting lines
• make Rect widths strictly positive (>= 0)

could also consider #181 here

Note: This is currently breaking for Makie