feat: impl forget exactly

[tisonkun]

Good for "resize" a semaphore.

From deadpool-rs/deadpool#147 (comment):

I implement a similar function today at #37 while notice that this try_acquire + forget can have a race condition to accidentally add available permits.

For example, there are pending semaphore.acquire()s inside timeout_get, and now a user call resize. try_acquire can return NoPermits because the pending acquires consume all available permits now. Then the less "total permits" of the semaphore gets reduced, causing the "total permits" larger than slots.max_size.