Add for statement and range function; New register_box_fn for functions returning Box<dyn Any> (which should not be boxed again); Register standard utility functions in library (e.g. print, string concat).

tests/for.rs

@@ -0,0 +1,24 @@
+use rhai::Engine;
+
+#[test]
+fn test_for() {
+    let mut engine = Engine::new();
+
+    let script = r"
+        let sum1 = 0;
+        let sum2 = 0;
+        let inputs = [1, 2, 3, 4, 5];
+
+        for x in inputs {
+            sum1 += x;
+        }
+
+        for x in range(1, 6) {
+            sum2 += x;
+        }
+
+        sum1 + sum2
+    ";
+
+    assert_eq!(engine.eval::<i64>(script).unwrap(), 30);
+}