auto merge of #7886 : msullivan/rust/default-methods, r=pcwalton

This does a bunch of cleanup on the data structures for the trait system. (Unfortunately it doesn't remove `provided_method_sources`. Maybe later.)

It also changes how cross crate methods are handled, so that information about them is exported in metadata, instead of having the methods regenerated by every crate that imports an impl.

r? @nikomatsakis, maybe?