A mailbox with an upper bound on the number of objects that can be simultaneously present.
Adding new elements may block when that number is reached.
The struct bounded-mailbox has a constructor make-bounded-mailbox that takes a capacity as a parameter.
Read-only accessor bounded-mailbox-semaphore refers to a semaphore that controls the number of elements that are present in the mailbox.
Read-only accessor bounded-mailbox-mailbox is the underlying actual mailbox.