Implement `Sync` for some windows sys types
Rename `UniquePtr` to `Unique`

Mostly following the convention in RFC 356
Relax `Arc` bounds don't require Sync+Send

Besides the above making sense, it'll also allow us to make `RacyCell`
private and use UnsafeCell instead.
Keep track of the whole error chain
Make Barrier and Condvar Sync/Send
Impl Sync/Send for Rawlink
Implement Sync/Send for ArcInner and Weak
Add a Racy type to bench tests