auto merge of #5196 : thestinger/rust/ord, r=catamorphism

This allows `TreeMap`/`TreeSet` to fully express their requirements and reduces the comparisons from ~1.5 per level to 1 which really helps for string keys.

I also added `ReverseIter` to the prelude exports because I forgot when I originally added it.

diff --cc src/libcore/prelude.rs
Simple merge

diff --cc src/libcore/str.rs
Simple merge

diff --cc src/libcore/vec.rs
Simple merge