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/dynamic_lib.rs

@@ -20,6 +20,7 @@ A simple wrapper over the platform's dynamic library facilities
 #![allow(missing_doc)]
 
 use clone::Clone;
+use collections::MutableSeq;
 use c_str::ToCStr;
 use iter::Iterator;
 use mem;