Define a function called sum-List that takes one List Nat argument and
evaluates to a Nat, the sum of the Nats in the list.
Define a function called maybe-last which takes (in addition to the type
argument for the list element) one (List E) argument and one E argument and
evaluates to an E with value of either the last element in the list, or the
value of the second argument if the list is empty.
Define a function called filter-list which takes (in addition to the type
argument for the list element) one (-> E Nat) argument representing a
predicate and one (List E) argument.
The function evaluates to a (List E) consisting of elements from the list
argument where the predicate is true.
Consider the predicate to be false for an element if it evaluates to zero, and true otherwise.
Define a function called sort-List-Nat which takes one (List Nat) argument
and evaluates to a (List Nat) consisting of the elements from the list
argument sorted in ascending order.