method resolution order

[nikomatsakis]

Both method dispatch (#73) and enum downcasting have a need to compute a "method resolution order". There is documentation in #73 suggesting we use Python's C3 algorithm, but in fact we do not. There is code for enum derive that uses a rather inefficient (but effective) algorithm to pick a good testing order. We should rework the Upcasts type to provide a proper MRO. This might be as simple as taking the code from enums, inefficient or not, I haven't thought too closely about it.

[rcoh]

@nikomatsakis is this done?