Rename `push/pop_back` to `push`/`pop`, add to `MutableSeq`

This fixes naming conventions for `push`/`pop` from either end of a structure by partially implementing @erickt's suggestion from https://github.com/rust-lang/rust/issues/10852#issuecomment-30823343, namely:

* push/pop from the 'back' are called `push` and `pop`.
* push/pop from the 'front' are called `push_front` and `pop_front`.
* `push`/`pop` are declared on the `MutableSeq` trait.
* Implement `MutableSeq` for `Vec`, `DList`, and `RingBuf`.
* Add `MutableSeq` to the prelude.

I did not make any further refactorings because there is some more extensive thought that needs to be put into the collections traits. This is an easy first step that should close https://github.com/rust-lang/rust/issues/10852.

I left the `push_back` and `pop_back` methods on `DList` and `RingBuf` deprecated. Because `MutableSeq` is in the prelude it shouldn't break many, but it is a breaking change.
As long as you're moving this code around, I'd love to see this rewritten in the third person singular present indicative, as is the dominant style (i.e. "Appends" instead of "Append"). Same with `fn push()`.