Make mapU/mapMonotonic and Prio.mapKeysMonotonic spine-strict

[treeowl]

I closed #95 because seqSpine is potentially useful to force the spine after mapU/mapMonotonic or Prio.mapKeysMonotonic. But thinking about it today, I realized that those operations should almost certainly be spine-strict anyway. There's almost nothing useful you can do to a queue without potentially forcing its spine. So I'd like to make those operations force the spine, and then bring #95 back with better documentation.

[treeowl]

I guess we should do this with all the other maps too. They're hideously inefficient, but I don't think this makes them much worse. The only way to make them reasonably efficient would be to make the binomial trees lazy, and I'm reluctant to go back to that; it opens up too many potential strictness bugs just to support operations that are likely rare.