Use parking_lot::Mutex instead of spin::Mutex

[ijl]

spin is no longer maintained.

Fixes #718.

[birkenfeld]

So the obvious question: why didn't the original use the std Mutex -- and is parking_lot the wrong choice for the same reason?

[ijl]

spin and parking_lot are both faster than std. This change is least likely to cause a difference for dependent projects. For anything more, putting energy into #679 would make these locks unnecessary.

[kngwyu]

@birkenfeld
I'm not the original author of this part and don't know the intention.
Both spin and parking_lot's documents say

No poisoning, the lock is released normally on panic.

So the difference from std is they return Error when panicking or not.
And... I honestly am not sure this feature matters, since I think most of the users don't write code where multiple threads try to get GIL 🤔

[kngwyu]

@ijl
Oh, I wrote the above comment before reading your comment.

spin and parking_lot are both faster than std.

I understand..., thanks.

[kngwyu]

Sorry, I want to think about std vs parking_lot problem more.
Please wait for a while.

[kngwyu]

How we should handle LockResult is not obvious, so I decided to use parking_lot for now.
However, maybe we should remove it in the near future.