Personally, I don't really *like* this naming, but hey, it's consistent with `Edge\Base::getVertexFromTo()` :) Perhaps `getEdgeBetween()`? But it's up to you, I'm okay with it either way.

The logic implements the same "get first element of" `$from->getEdgesTo($to)`, right? :) Once #48 lands, this should probably be simplified as `$from->getEdgesTo($to)->getEdgeFirst()`.