In resolve, re-use ribs to avoid allocation

[catamorphism]

As per comment in resolve::Resolver

[pnkfelix]

The original fixme comment was introduced in f093d37 (in the file resolve3.rs in that commit; github does not show it for me in the commit, you need to do view file, or not use github. :)

From looking at the resolve code, it seems like we indeed push and pop rib objects in a pretty regular fashion, and may also only use it like a stack... (maybe fixing this is actually easy; I do not immediately see a case where the @Rib elements of value_ribs escape from their containing vector...)

[alexcrichton]

I believe resolve has changed quite a bit since this was opened, and I think it's best to track the implementation details of resolve inside of resolve itself to make sure they stay up to date.

[nagisa]

This issue is still relevant. That is, we still do allocations (pushes) and deallocations (pops). Maybe we should move from vector to some implementation of stack instead (not that it would decrease amount of allocations much)?