fix(maitake): handle spurious `WaitCell` polls

[hawkw]

This branch changes the Wait future for maitake::sync::WaitCell to handle spurious polls correctly. Currently, a wait_cell::Wait future assumes that if it's ever polled a second time, that means its waker was woken. However, there might be other reasons that a stack of futures containing a Wait is polled again, and the Wait future will incorrectly complete immediately in that case.

This branch fixes this by replacing the bool field in Wait that's set on first poll with an "event count" stored in the remaining WaitCell state bits. Now, when a Wait is created, it loads the current event count, and calls to wake() and close() increment the event count. The Wait future then checks if the event count has gone up when it's polled, rather than just checking if it's ever been polled before. This allows the Wait future to determine if it is being polled because the WaitCell woke it up, or if it's being polled because some other future decided to poll it. This also has the side benefit of fixing racy scenarios where the WaitCell is woken between when the Wait future is created and when it's polled for the first time.

Fixes #449

[hawkw]

@jamesmunns it would be nice to confirm that this actually fixes the weird behavior you were seeing in mnemOS...