Introduce range-over-func iter.Seq method All() for graph.EndpointPair

[jbduncan]

Blocked on the currently-experimental range-over-func Go feature being merged into a stable version of Go.

This will allow the nodes of an endpoint pair to be iterated on with for loops, which is more flexible than introducing a ForEach method which may not allow early exits from the loop. Furthermore, it supports turning the nodes into a "pull iterator" with the proposed iter.Pull, which lets user control when the iterations happen.

[jbduncan]

For clarity, this would treat graph.EndpointPair as a two-element collection with a source and target node that the iterator would return.