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/ascii.rs

@@ -19,7 +19,7 @@ use fmt;
 use iter::Iterator;
 use mem;
 use option::{Option, Some, None};
-use slice::{ImmutableSlice, MutableSlice, Vector};
+use slice::{ImmutableSlice, MutableSlice, Slice};
 use str::{Str, StrSlice};
 use str;
 use string::String;