Fix scoping and break statements.

Variables declared in the loop were not properly disposed of
when you broke out of the loop.

test/for/close_over_loop_variable.wren

@@ -2,7 +2,6 @@ var list = []
 
 for (i in [1, 2, 3]) {
   list.add(fn IO.print(i))
-  //list.add(fn IO.print(i))
 }
 
 for (f in list) f.call

test/for/return_closure.wren

@@ -0,0 +1,9 @@
+var f = fn {
+  for (i in [1, 2, 3]) {
+    return fn IO.print(i)
+  }
+}
+
+var g = f.call
+g.call
+// expect: 1

test/for/return_inside.wren

@@ -0,0 +1,8 @@
+var f = fn {
+  for (i in [1, 2, 3]) {
+    return i
+  }
+}
+
+IO.print(f.call)
+// expect: 1

test/for/syntax.wren

@@ -18,3 +18,6 @@ for
 // expect: 1
 // expect: 2
 // expect: 3
+
+// TODO: Return from inside for loop.
+// TODO: Return closure from inside for loop.