InvariantLifetime is Copy-able

Both ContravariantLifetime and CovariantLifetime are marked as Copy,
so it makes sense for InvariantLifetime to be as well.

src/libcore/kinds.rs

@@ -259,6 +259,8 @@ pub mod marker {
     #[deriving(Clone, PartialEq, Eq, PartialOrd, Ord)]
     pub struct InvariantLifetime<'a>;
 
+    impl<'a> Copy for InvariantLifetime<'a> {}
+
     /// A type which is considered "not sendable", meaning that it cannot
     /// be safely sent between tasks, even if it is owned. This is
     /// typically embedded in other types, such as `Gc`, to ensure that