std: Rename slice::Vector to Slice

This required some contortions because importing both raw::Slice
and slice::Slice makes rustc crash.

Since `Slice` is in the prelude, this renaming is unlikely to
casue breakage.

[breaking-change]

src/libcore/prelude.rs

@@ -63,4 +63,4 @@ pub use tuple::{Tuple5, Tuple6, Tuple7, Tuple8};
 pub use tuple::{Tuple9, Tuple10, Tuple11, Tuple12};
 pub use slice::{ImmutableEqSlice, ImmutableOrdSlice};
 pub use slice::{MutableSlice};
-pub use slice::{Vector, ImmutableSlice};
+pub use slice::{Slice, ImmutableSlice};