Move to parking_lot::Mutex for mutex and away from std::sync::Mutex?

[bluejekyll]

parking_lot has a lot of advantages over std::sync::Mutex.

Should we migrate to parking_lot for all futures_util::lock primitives?

If we're open to this, I'd be happy to contribute the work for this.

[taiki-e]

I would accept a PR to add an optional feature to do this.
(parking_lot is very good, but note that it may fail to compile on some platforms. futures needs to work on such platforms. e.g., Amanieu/parking_lot#269)

[lilyball]

I would love to see a parking_lot feature. I keep ending up taking code using parking_lot::Mutex and changing it to futures::lock::Mutex because it's being used in an async context, and this feels like a regression.

[ibraheemdev]

Given the work going on to improve std::sync::Mutex, it might not be of much benefit to add this now.

[taiki-e]

I agreed with @ibraheemdev, and tend to close this for now.

relevant discussion: tokio-rs/tokio#4623