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/os.rs

@@ -45,7 +45,7 @@ use path::{Path, GenericPath, BytesContainer};
 use ptr::RawPtr;
 use ptr;
 use result::{Err, Ok, Result};
-use slice::{Vector, ImmutableSlice, MutableSlice, ImmutableEqSlice};
+use slice::{Slice, ImmutableSlice, MutableSlice, ImmutableEqSlice};
 use str::{Str, StrSlice, StrAllocating};
 use string::String;
 use sync::atomic::{AtomicInt, INIT_ATOMIC_INT, SeqCst};