[FIX] qweb: do not leak values in global context in rare cases

In some very rare cases (such as the use of the t-foreach directive),
Owl did leak the values in the render context in the global context.
This was due to the fact that the compiled template looked like this:

let _3 = _4 = _5;

instead of

let _3 = _5;
let _4 = _5;

tests/qweb/qweb_memory.test.ts

@@ -0,0 +1,17 @@
+import { QWeb } from "../../src/qweb/index";
+import { renderToString } from "../helpers";
+
+describe("memory", () => {
+  test("t-foreach does not leak stuff in global scope", () => {
+    let qweb = new QWeb();
+    const initialNumberOfGlobals = Object.keys(window).length;
+    qweb.addTemplate(
+      "test",
+      `<p><t t-foreach="[3, 2, 1]" t-as="item"><t t-esc="item"/></t></p>`
+    );
+    const result = renderToString(qweb, "test");
+    const expected = `<p>321</p>`;
+    expect(result).toBe(expected);
+    expect(Object.keys(window).length).toBe(initialNumberOfGlobals);
+  });
+});