[fine] Support assignment to member, loops with iterators

Hmm it's starting to look like something.

fine/tests/expression/loops.fine

@@ -1,3 +1,21 @@
+class Finished {}
+let FINISHED = new Finished {};
+
+class Iterator {
+  current: f64;
+
+  fun next(self) -> f64 or Finished {
+    if self.current < 10 {
+      let result = self.current;
+      self.current = self.current + 1;
+      return result;
+    }
+
+    FINISHED
+  }
+}
+
+
 fun test() -> f64 {
   let result = 1;
   let i = 0;
@@ -5,8 +23,15 @@ fun test() -> f64 {
     result = result * 2;
     i = i + 1;
   }
-  result
+
+  let sum = 0;
+  let it = new Iterator { current: 0 };
+  while it.next() is v: f64 {
+    sum = sum + v;
+  }
+
+  result + sum
 }
 
 // @no-errors
-// @eval: Float(1024.0)
+// @eval: Float(1069.0)