collections: Move push/pop to MutableSeq

Implement for Vec, DList, RingBuf. Add MutableSeq to the prelude.

Since the collections traits are in the prelude most consumers of
these methods will continue to work without change.

[breaking-change]

src/libstd/lib.rs

@@ -288,4 +288,6 @@ mod std {
     #[cfg(test)] pub use os = realstd::os;
     // The test runner requires std::slice::Vector, so re-export std::slice just for it.
     #[cfg(test)] pub use slice;
+
+    pub use collections; // vec!() uses MutableSeq
 }