Add Semaphore::is_closed method

[bikeshedder]

Motivation

Right now the only way to check if a semaphore is closed is to do the following:

let is_closed = matches!(
    semaphore.try_acquire_many(0),
    Err(TryAcquireError::Closed)
);

It would be nice if I could just call semaphore.is_closed() instead.

Solution

Add a is_closed method to the public Semaphore API which calls the is_closed method of the underlying low level semaphore.

[Darksonn]

Thanks!