Write a function with the following type:
val longsub : 'a list -> ('a * int) option = <fun>such that longsub l evaluates to None if the list l is empty,
otherwise it evaluates to the pair Some(x,n) if the longest subsequence
of equal elements in l has length n, and contains the element x.
For example:
longsub [1;1;3;3];;
- : (int * int) option = Some (1, 2)
longsub [1;2;2;2;1;1];;
- : (int * int) option = Some (2, 3)
longsub [1;1;2;2;2;1;1;1;1];;
- : (int * int) option = Some (1, 4)