[IMP] qweb: turn handlers into function expressions only

For the sake of consistency with vanilla JS, and to allow some things
that were previously not possible.

tests/components/__snapshots__/refs.test.ts.snap

@@ -43,7 +43,8 @@ exports[`refs refs are properly bound in slots 2`] = `
   
   const slot3 = ctx => (node, key) => {
     const refs = ctx.__owl__.refs
-    let d2 = [ctx, 'doSomething'];
+    const v4 = ctx['doSomething'];
+    let d2 = [v4, ctx];
     let d3 = (el) => refs[\`myButton\`] = el;
     return block2([d2, d3]);
   }