Add support for subsets of 3x3 solving

[ishaanbuildsthings]

Give the user the option to solve certain aspects of the cube, such as f2l, OLL, etc. This will require slightly modifying the search algorithm and cube model to use black stickers which represent non-unique cases. For instance building a solver to solve OLL will require putting black stickers on the top layer outer bands, so that we are indifferent to solving the permutations.