feature request：support cancellable RwLock APIs

[sammyne]

For example,

pub fn RwLockUpgradableReadGuardArc::upgrade(guard: Self) -> UpgradeArc<T>

implemented as

pub fn RwLockUpgradableReadGuardArc::upgrade(guard: Self, cancel: impl Future<Output=()>) -> UpgradeArc<T>

When cancel return Ready, just return UpgradeArc untouched.

[notgull]

I like the idea in theory but in practice the API would need a better design.