MergeResult.Success if merge was successful.
MergeResult.WereAlreadyEqual if merge was already performed.
MergeResult.IdClash if two identical ids were found (should never allow to happen).
MergeResult.Incompatible if widths of this and other are not equal.
Progress: Lock-free.
Attempts to merge this PriorityQueue with another of the same width.
After a successful merge, this and other will both be handles refering to the same width InnerPQs, where the two original InnerPQs with the same [0,width-1] index will be merged to create the InnerPQ of that index in the result.