Given a pivot x, and a list lst, partition the list into three parts.
- The first part contains all elements in lst that are less than x
- The second part contains all elements in lst that are equal to x
- The third part contains all elements in lst that are larger than x
Ordering within a part can be arbitrary.
For example, given x = 10 and lst = [9, 12, 3, 5, 14, 10, 10], one partition may be `[9, 3, 5, 10, 10, 12, 14].