Introduce Arc and interfaces.

Still lots of conversion to do.
Updated basic directed variants
Add undirected variant.
Add data, weighted, and labeled variants.
Add EachArc() to the literal fixture
Fix uses of Source()/Target() in the spec tests.
Fixes to shared/entry funcs for adjacency lists
Bring Bernoulli rand graphs up to speed with Arcs.
goimports -w
Make the arc enumerators use ArcSteps.
Update spec tests with in/out arc enums
Update dfs with arcs
Update als with in/out arc use of ArcStep
Shut the stupid defunct bench test up temporarily
Introduce ArcLists, convert fixtures to them
Get all als passing again.

Some tests seem to be missing, though...
Start granulating test suites.

This entails both separate files, and testing by the atomic interfaces
instead of the composites.
Forgot several RemoveEdges -> RemoveArcs
TESTS GREEN. (Still much to do)
Add Digraph.HasArc().

Slippery slope here, would be nice to not explode with yet more methods