Array reverse helper (#1805)

Really small. Just thought might not be appropriate to put in Keccak.

std/array.asm

@@ -15,6 +15,9 @@ let<T> set_element: T[], int, T -> T[] = |arr, i, x| {
 /// Returns a new array of length l containing the elements of arr starting at index start.
 let<T> sub_array: T[], int, int -> T[] = |arr, start, l| std::array::new(l, |i| arr[start + i]);
 
+/// Returns a new array that reverses the input array.
+let<T> reverse: T[] -> T[] = |arr| new(len(arr), |i| arr[len(arr) - i - 1]);
+
 /// Evaluates to the array [f(arr[0]), f(arr[1]), ..., f(arr[len(arr) - 1])].
 let<T1, T2> map: T1[], (T1 -> T2) -> T2[] = |arr, f| new(len(arr), |i| f(arr[i]));