Add "proper coloring" unittest check

We have to check that for each edge its vertices have different colors.

This is very easy to do with a vertex-to-color map, but not so easy with
a color-to-vertex one. Since all our coloring algorithms created a
vertex-to-color map behind the scenes and then converted it, we flip
them back to returning it directly, and do the conversion explicitly
where we need it (which for now is everywhere except when testing this
property).

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>