Initial implementation of list-based color mixing.
Initial implementation of list-based color mixing.

This isn't switched over yet, but the implementation is in place for
when it is.  See #768

alphalambda/codeworld,pranjaltale16/codeworld,google/codeworld,alphalambda/codeworld,pranjaltale16/codeworld,google/codeworld,alphalambda/codeworld,google/codeworld,pranjaltale16/codeworld