Greedy coloring for graphs/digraphs

[danshapero]

This PR includes a greedy coloring algorithm using the first-fit heuristic and some accompanying tests.

[aysylu]

Thanks, this looks great! Could you please add just 1 failure test case for greedy-coloring? So that we can document what the failure case returns. I'll merge once the test case is in.

[danshapero]

Hm, I'm not quite sure what you have in mind -- could you elaborate?

The greedy-coloring function should (I hope!) produce a valid coloring for any graph, although it might use more colors than the graph's chromatic number. I didn't want to check for specific output from greedy-coloring, because that'll depend on the unimportant detail of the order in which you visit all the vertices. The coloring? predicate is there to check validity of the output instead, but I test that function against specific colorings I did by hand for some of your test graphs.

[aysylu]

Sorry, what I meant is it'd be nice to show that on a simple "triangle" graph greedy-coloring produces 3 colors. I'm merging the PR as is and will add the test myself.

Thanks for your contribution!