Fix potential undefined behavior in ArrayQueue initialization

[caelunshun]

As far as I can tell, ArrayQueue::new() would previously create temporary references
to uninitialized data, which is undefined behavior. The stamps for the Slot buffer were initialized like so:

// Initialize stamps in the slots.
for i in 0..cap {
    unsafe {
        // Set the stamp to `{ lap: 0, index: i }`.
        let slot = buffer.add(i);
        ptr::write(&mut (*slot).stamp, AtomicUsize::new(i));
    }
}

As I see it, the code above creates a temporary reference to the uninitialized Slots on the call to ptr::write(), equivalent to the following:

for i in 0..cap {
    unsafe {
        // Undefined behavior: reference to uninitialized memory
        let slot: &mut Slot<T> = &mut *buffer.add(i);
        // ... 
    }
}

The problem was fixed by creating the slots vector using Iterator::collect(). This has the extra benefit of removing an unnecessary unsafe block.

The same issue was present in the bounded flavor for crossbeam-channel, to which I applied the same change.

For reference, see the nomicon:

For a struct, however, in general we do not know how it is laid out, and we also cannot use &mut base_ptr.field as that would be creating a reference. Thus, it is currently not possible to create a raw pointer to a field of a partially initialized struct, and also not possible to initialize a single field of a partially initialized struct. (A solution to this problem is being worked on.)

[ghost]

Thank you!