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/libstd/dynamic_lib.rs

@@ -29,7 +29,7 @@ use option::*;
 use os;
 use path::{Path,GenericPath};
 use result::*;
-use slice::{Vector,ImmutableSlice};
+use slice::{Slice,ImmutableSlice};
 use str;
 use string::String;
 use vec::Vec;