Add `UnionEdgeGraph`, `Either`, `ConcatenatedCollection` and `MutableGraph.addEdges`#84

Open

saeta wants to merge 14 commits intomainfrom

Commits on Jun 15, 2020

Begin adding a union-edges composite graph, but being forced into defining either to avoid reimplementing ConcatenatedCollection a bunch of times.
saeta
committed
Augment `addEdges` to take a callback and add tests.
saeta
committed
Add either tests.
saeta
committed

Commits on Jun 16, 2020

Commits on Jun 19, 2020

WIP: Respond to comments
saeta
committed

Commits on Jun 20, 2020

Commits on Jun 21, 2020

Merge remote-tracking branch 'origin/master' into union-edges
saeta
committed

Commits on Jul 4, 2020