Optimize shim generation for structural items

This commit removes shims, where possible, for `structural` items.
Instead of generating code that looks like:

    const target = function() { this.foo(); };
    exports.__wbg_thing = function(a) { target.call(getObject(a)); };

we now instead generate:

    exports.__wbg_thing = function(a) { getObject(a).foo(); };

Note that this only applies to `structural` bindings, all default
bindings (as of this commit) are still using imported targets to ensure
that their binding can't change after instantiation.

This change was [detailed in RFC #5][link] as an important optimization
for `structural` bindings to ensure they've got performance parity with
today's non-`structural` default bindings.

[link]: https://rustwasm.github.io/rfcs/005-structural-and-deref.html#why-is-it-ok-to-make-structural-the-default

crates/js-sys/tests/wasm/Object.rs

@@ -129,7 +129,7 @@ fn get_own_property_descriptor() {
     let desc = Object::get_own_property_descriptor(&foo, &"foo".into());
     assert_eq!(PropertyDescriptor::from(desc).value(), 42);
     let desc = Object::get_own_property_descriptor(&foo, &"bar".into());
-    assert!(PropertyDescriptor::from(desc).value().is_undefined());
+    assert!(desc.is_undefined());
 }
 
 #[wasm_bindgen_test]

crates/js-sys/tests/wasm/Reflect.rs

@@ -114,7 +114,7 @@ fn get_own_property_descriptor() {
     let desc = Reflect::get_own_property_descriptor(&obj, &"x".into()).unwrap();
     assert_eq!(PropertyDescriptor::from(desc).value(), 10);
     let desc = Reflect::get_own_property_descriptor(&obj, &"foo".into()).unwrap();
-    assert!(PropertyDescriptor::from(desc).value().is_undefined());
+    assert!(desc.is_undefined());
 }
 
 #[wasm_bindgen_test]