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/io/mod.rs

@@ -235,7 +235,7 @@ use os;
 use boxed::Box;
 use result::{Ok, Err, Result};
 use rt::rtio;
-use slice::{Vector, MutableSlice, ImmutableSlice};
+use slice::{Slice, MutableSlice, ImmutableSlice};
 use str::{Str, StrSlice};
 use str;
 use string::String;