Optimize generated code

There are numerous places where we can improve the generated code.

Example:

```cs
[MonoPInvokeCallback (typeof (DQLPreviewReplyDataCreationHandler))]
static unsafe IntPtr Invoke (IntPtr block, IntPtr reply, ref IntPtr error) {
var descriptor = (BlockLiteral *) block;
var del = (global::QuickLook.QLPreviewReplyDataCreationHandler) (descriptor->Target);
var __xamarin_pref1 = Runtime.GetINativeObject (error, false);
Foundation.NSData retval = del ( Runtime.GetNSObject (reply), ref __xamarin_pref1);
error = __xamarin_pref1 is null ? IntPtr.Zero : __xamarin_pref1.Handle;
return retval.GetHandle ();
}
```

* `Runtime.GetINativeObject` should be `GetNSObject`
* `error = __xamarin_pref1 is null ? IntPtr.Zero : __xamarin_pref1.Handle;` should be `error = `__xamarin_pref1.GetHandle ()`;

We should look at the generated code to find more opportunities.