task: use pin-project for TaskLocalFuture

[BugenZhao]

Motivation

As pin-project-lite supported custom Drop implementation in taiki-e/pin-project-lite#25, I suppose it's worthwhile switching back to the pin-project implementation to eliminate unsafe code for TaskLocalFuture.

Solution

Implement TaskLocalFuture with pin_project_lite::pin_project! with PinnedDrop.

[BugenZhao]

I'm not really clear about whether we need this Future to be !Unpin, as making it Unpin seems to make life easier in some situations.

For example, if the feature in #5743 is added, it'll be common that we poll this with &mut fut to avoid the future being consumed, then we're able to take the value back after getting Ready. If the future is always !Unpin, we are not able to take back the ownership of the future. :(

Currently, I leave it untouched according to #3943 (comment). IIUC, this is considered to be a breaking change in the compatibility?

[Darksonn]

Removing the PhantomPinned field would not be breaking, but we could never undo it because that would be breaking.

As for into_value, you make a good point. That method would not be able to take ownership of the future.

[taiki-e]

Thanks!