Reexport static methods on structs & enums.

This fixes #6745, which itself relates to #4202. Slightly ham-fisted -- feel particularly funny about using the typeck phase to gather the base -> impl mapping, and the separate code paths for traits vs. "real" bases feels like it could be avoided -- but it seems to work.

As always, open to suggestions if there's a better way to accomplish what I'm trying to do.

@catamorphism r?