use slots for ParkingLot and Event

[belm0]

For the wait queue use case, ParkingLot is efficient and doesn't generate garbage.

However if a wait queue is needed on anyio, you'd have to use Event and create a new instance on each set(). Event doesn't use slots, and contains ParkingLot which also doesn't use slots. So that's two unslotted class instances per set().

Any reason not to use slots on these classes?

[belm0]

Event was made more efficient and no longer depends on ParkingLot.

I forgot to have ParkingLot use slots in the PR, but that's relatively minor now-- for another day.