Fix scoping and break statements.

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

test/break/exit_local_scopes.wren

@@ -0,0 +1,18 @@
+for (i in 0..10) {
+  IO.print(i)
+
+  {
+    var a = "a"
+    {
+      var b = "b"
+      {
+        var c = "c"
+        if (i > 1) break
+      }
+    }
+  }
+}
+
+// expect: 0
+// expect: 1
+// expect: 2