s/eachAdjacent()/eachAdjacentTo()/g
Update test dependencies
Stop emitting edge on eachAdjacentTo().
Make AdjacencyList a trait; first step to spec testing.
Move undirected al test entirely into GraphSpec
s/DirectedGraph/Digraph/
Granulate MutableGraph into MutableVertexSet
Make test autoloader a little more robust
s/addDirectedEdge()/addArc()/
Split MutableGraph from MutableDigraph.
s/MutableDigraph::removeEdge()/removeArc()/
s/add{Edge,Arc,Vertex}()/ensure{Edge,Arc,Vertex}()/
Some follow-on renaming to the add/ensure thing
Clean up and solidify tests some more.
Drop "each" prefix from enumeration methods.
Fix the mess that was *degree().

Add -Of suffix to method name for consistency and clarity. Much more
importantly, move in/out degree to Digraph interface only, and add
degreeOf() to Graph.
Introduce Graph::incidentTo().
Introduce successorsOf(), predecessorsOf() on Digraph.

This also meant finally fixing the incorrect implementation of adjacentTo on digraphs.
Fixed composer.lock
Fixed .gitignore