Fix scoping and break statements.

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

test/while/return_closure.wren

@@ -0,0 +1,10 @@
+var f = fn {
+  while (true) {
+    var i = "i"
+    return fn IO.print(i)
+  }
+}
+
+var g = f.call
+g.call
+// expect: i

test/while/return_inside.wren

@@ -0,0 +1,9 @@
+var f = fn {
+  while (true) {
+    var i = "i"
+    return i
+  }
+}
+
+IO.print(f.call)
+// expect: i