allow destructuring tuples when assigning

examples/fib.ang

@@ -1,8 +1,21 @@
 
 # calculate fibonacci numbers with a loop
 
-(a, b) := (1, 0)
-for _ in 0..100 {
-    (a, b) = (a + b, b)
+fn range(from: int, to: int) -> (fn() -> (int, bool)) {
+    i := from - 1
+    end := to - 1
+
+    fn() -> (int, bool) {
+        if i < end {
+            (i = i+1, true)
+        } else {
+            (-1, false)
+        }
+    }
+}
+
+(a, b) := (0, 1)
+for _ in range(0, 100) {
+    (a, b) = (a + b, a)
     println(a)
 }