Option map

Write a function option_map that takes a function f and an option value, and returns:

None, if the input value is None;
Some (f x), if the input value is Some x.

The function must have the following type:

option_map : ('a -> 'b) -> 'a option -> 'b option

Here are some unit tests:

let double x = x * 2;;
let square x = x * x;;

assert (option_map double (Some 3) = Some 6);;
assert (option_map double None = None);;
assert (option_map double (Some (square 3)) = Some 18);;
assert (option_map square (option_map double None) = None);;