O(\log n). Return the index of an element, which is its zero-based
index in the sorted sequence of elements. The index is a number from 0 up
to, but not including, the size of the set. Calls error when the element
is not a member of the set.
findIndex 2 (fromList [5,3]) Error: element is not in the set
findIndex 3 (fromList [5,3]) == 0
findIndex 5 (fromList [5,3]) == 1
findIndex 6 (fromList [5,3]) Error: element is not in the set