std: Add Vec to the prelude

This is an incredibly common type, and it is expected to be used in many many
places. This type should be in the prelude.

src/libstd/prelude.rs

@@ -59,6 +59,7 @@ pub use slice::{ImmutableEqVector, ImmutableTotalOrdVector, ImmutableCloneableVe
 pub use slice::{OwnedVector, OwnedCloneableVector, OwnedEqVector};
 pub use slice::{MutableVector, MutableTotalOrdVector};
 pub use slice::{Vector, VectorVector, CloneableVector, ImmutableVector};
+pub use vec::Vec;
 
 // Reexported runtime types
 pub use comm::{channel, Sender, Receiver};