Add a new method_super origin for supertrait methods. Also make
coherence create a table that maps pairs of trait IDs and self types
to impl IDs, so that it's possible to check a supertrait method
knowing only its index in its trait's methods (without knowing all
supertraits for a given trait).
r=nmatsakis and graydon -- with hope, we'll revamp all of this code as
per #4678, but for now this fixes the bug.
Closes#3979
