Adds a merge() method to:

    * SemaphorePermit
    * OwnedSemaphorePermit

Merging two permits instances together consumes one instance, adding the
permits it holds to the remaining instance.

Prevent merging permits from two different Semaphore instances.
Attempting to merge unrelated permits will panic.

Document merging permits from distinct Semaphore instances causes a
panic, and add #[track_caller] annotation.