Write a polymorphic function that returns the first, third and fifth element of a list. Its type must be:
first_third_fifth : 'a list -> ('a * 'a * 'a) option If the list is not long enough, the result is None, otherwise it is a triple of values wrapped in the constructor Some.
For example:
first_third_fifth ["cat"; "dog"] = None
first_third_fifth [1; 2; 3; 4; 5; 6] = Some (1, 3, 5)