Use parking_lot sync primitives

[jzajpt]

Use Mutex, RwLock and others from parking_lot crate instead of std::sync

[Pauan]

Thanks, but there's a few issues with using parking_lot:

1. For usage on Wasm, you have to use the nightly compiler and enable the wasm-bindgen feature, which is quite annoying (and a breaking change for all Wasm consumers).
2. There are issues with using parking_lot on Wasm.
3. The stdlib implementation for Mutex and RwLock on Wasm should be quite a lot faster than parking_lot.
4. There are plans to implement parking_lot directly in the stdlib, though unfortunately it seems those plans have stalled.

So if we're going to implement parking_lot, it would have to be as an optional dependency (which also means using cfg to choose the right implementation at compile time). That's a quite large change.

I am aware of the speed benefits of parking_lot (which is why the code has a TODO about it), but are the performance benefits worth the above problems? I'm not sure.