Fix build (obsolete tilde)

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

I think in this case it would be better to use a `vec![]` and the `as_mut_slice()` method to be more consistent with real-world code (usually you don't see something like `&mut[]` and I guess this can be strange for beginners). However, this is a matter of taste, so if you disagree I will merge it anyway.