Misc Stabilization for collections

This pull request:

*Renames `BinaryHeap::top` to `BinaryHeap::peek`
*Stabilizes `front/back/front_mut/back_mut` in `DList` and `RingBuf`
*Stabilizes `swap` in `RingBuf`

in accordance with rust-lang/rfcs#509.

Note that this PR does not address `Bitv::{get,set}` or HashMap's iterators, nor does it move `std::vec` to `std::collections::vec`, all of which still need to be done.

Because of the method renaming, this is a [breaking-change].