Compute a method resolution order

Python orders a class's ancestors with the C3 linearization. Write mro(name, bases) that computes it from a table, so you can check a class hierarchy before writing it. bases maps each class name to the list of its direct bases, in order ("object" maps to []). The MRO of a class is the class itself followed by the merge of its bases' MROs and the list of its bases. To merge: repeatedly take the first head (first item) of the lists that doesn't appear in the *tail* (anything after the first item) of any list, append it, and remove it from the front of every list; if no head qualifies, the hierarchy is impossible.