Auto merge of #45267 - oconnor663:rwlock_send, r=alexcrichton
Auto merge of #45267 - oconnor663:rwlock_send, r=alexcrichton

remove the `T: Sync` requirement for `RwLock<T>: Send`

That requirement makes sense for containers like `Arc` that don't
uniquely own their contents, but `RwLock` is not one of those.

This restriction was added in https://github.com/rust-lang/rust/commit/380d23b5d4b9fb8f5f0ebf178590f61528b2483e, but it's not clear why. @hniksic
and I [were discussing this on reddit](https://www.reddit.com/r/rust/comments/763o7r/blog_posts_introducing_lockfree_rust_comparing/dobcvbm/). I might be totally wrong about this change being sound, but I'm super curious to find out :)

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