Fix build (obsolete tilde)

~[int] is now obsoleted
also sync::Future is now in std::sync::Future

should fix the build

@aochagavia I have no strong feelings in a way or another. I guess it just looked wasteful to allocate an empty vector on the heap just to take a slice of it, but you're right that (besides this specific case, when you just want to test what happens with an empty vector) I can't really think of anything you can do with an &mut[] in real code, as I don't think you can really mutate anything...