Write a function atoi : string -> int (i.e., "ASCII to integer") such that a call of the function such as (atoi "59") would return the integer 59. You may assume that the characters in the string are all decimal digits. Hint: Consider using the function Lib.explode : string -> char list. For example, (Lib.explode "314") returns the list of characters ['3'; '1'; '4'].