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.
I recall this had something to do with the 'multiple-std' testing problem. The test cases where trying to use the methods defined on `Vec` in std while these test cases are trying to test the `Vec` defined in the collections crate under test. This extra import imports the correct version of `MutableSeq` and makes the method resolve correctly.