Missing `A: Sync` bound for `Allocator` for `vec::IntoIter`

[steffahn]

I think this is an issue? Especially in light of IntoIter::allocating(&self) -> A existing. If so, the fix is trivial.

The instance in question: https://doc.rust-lang.org/nightly/std/vec/struct.IntoIter.html#impl-Sync

For comparison, the Send implementation right next to it does feature A: Send.