Coloring results

[gdalle]

At the moment, ADTypes specifies that the result of coloring is just a vector of integer colors.
However, in the case of symmetric coloring, we can compute other things (a star set) that speed up decompression.
More generally, a coloring algorithm could return an AbstractColoringResult, which is then used in a decompression function whose API we need to hash out. Perhaps #35 can help?