Question: Can WakerSet be Send+Sync?

[WakerSet](https://github.com/async-rs/async-std/blob/master/src/sync/waker_set.rs) implements its own atomic locking mechanism but is not Send or Sync. The use of it in async-std [is accompanied by](https://github.com/async-rs/async-std/blob/master/src/sync/condvar.rs#L63-L64) unsafe impl Send and Sync for containing types, but that seems like it would be more appropriate to put on the type that provides the locking mechanism for locality of reference. Is there a reason that WakerSet isn't Send + Sync but Condvar is?

I'd happily open a PR to move the unsafe impl Send and Sync to WakerSet unless I've misunderstood something about the safety of this type