[FIX] qweb/component: refactoring of scoping/variables/slots

This commit is a significant refactoring of the internal of QWeb. It
simplifies the way variables/scoping and slots interact together.  The
main idea is that we use a simple scope object instead of a
context/var/scope object.

This commit also implement the actual correct QWeb semantic for the
t-call directive with a sub body.  Before, we simply extracted the
variables from the body and injected them at the top of the sub
template.  We now simply compile the body before the sub template.

This is a joint work with Lucas (lpe).

closes #541
closes #544
closes #545

closes #557
closes #556

tests/tooling/debug_script_3.test.ts

@@ -23,7 +23,7 @@ test("log a specific message for render method calls if component is not mounted
 
   class Parent extends Component<any, any> {
     static template = xml`<div><t t-esc="state.value"/></div>`;
-    state = owl.hooks.useState({value: 1});
+    state = owl.hooks.useState({ value: 1 });
   }
 
   const parent = new Parent(null, {});