Add Array.size (#494)

This aims to address https://forum.dfinity.org/t/how-to-get-a-array-lengh/8346 and other instances of people asking about how to get the size of an array due to it being omitted from the generated documentation.

Co-authored-by: Paul Young <84700+paulyoung@users.noreply.github.com>

src/Array.mo

@@ -697,5 +697,21 @@ module {
   /// Runtime: O(1)
   ///
   /// Space: O(1)
-  public func keys<X>(array : [X]) : I.Iter<Nat> = array.keys()
+  public func keys<X>(array : [X]) : I.Iter<Nat> = array.keys();
+
+  /// Returns the size of `array`.
+  ///
+  /// NOTE: You can also use `array.size()` instead of this function. See example
+  /// below.
+  ///
+  /// ```motoko include=import
+  ///
+  /// let array = [10, 11, 12];
+  /// let size = Array.size(array);
+  /// ```
+  ///
+  /// Runtime: O(1)
+  ///
+  /// Space: O(1)
+  public func size<X>(array : [X]) : Nat = array.size()
 }