Add stackPop

Not sure if popping an empty stack should be undefined, empty stack, or
an error. I went with empty stack for now, for simplicity.
Add sortDel

This gets weirder still. I can't really pop from something that's been
sorted, so I'll have to just get the head of the stack list, then delete
that value from the sort list. It is still deleting in a stack-based
way, if you stretch things a bit; it deletes the first occurrence (i.e.
the closest occurrence to the head of the list), which should be the one
added most recently, meaning this is a 'stable' system.
Add stackSize and size
Add stackHead

Pop will require first popping from the stack list, then using that
value to delete from the sorted list. However, that value is not
returned by pop, because this is an immutable system, and I return the
smaller list, not the popped value, which must be gotten separately.
Add pop

This is complicated, but then, so is this corny data structure.
Add stackValues, sortValues

I was thinking I'd use these for removeGreater.