Fix Dsatur and add Dcolor

Our Dsatur implementation was incorrect: while the paper defined the
degree of saturation of a vertex as the number of different colors it is
adjacent to, we were using the number of colors, without considering
uniqueness. This effectively implemented a different algorithm, which is
very similar to the previous one, and while it performs slightly worse
on average it still beats Dsatur on some cases.

So we refactor the implementation to effectively support both algorithms
without code duplication, and then we export both the old algorithms as
"Dcolor" and the new one as "Dsatur". Since these are all fast
algorithms in hroller we will still be able to pick the best result.

Note that the new Dsatur implementation uses an IntSet to calculate the
uniqueness. Results with nub + length on a list were significantly
slower.

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