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/os.rs

@@ -32,7 +32,7 @@
 #![allow(non_snake_case_functions)]
 
 use clone::Clone;
-use collections::Collection;
+use collections::{Collection, MutableSeq};
 use fmt;
 use io::{IoResult, IoError};
 use iter::Iterator;