Support non-literal-closures for VTag event handlers

[hgzimmerman]

Description

I'm submitting a bug report/feature

When creating callbacks inside the html! macro, you can use tuple-function syntax (eg: callback=Some if Option<_> is the component's message type) instead of providing a normal closure (eg callback=|x| Some(x)) when you create components.
For some reason, this isn't possible for event handlers in Vtags. where oninput=Some will cause the html! macro to complain about it expecting a closure in the form of |_| Etc...

I don't see an immediate reason why this syntax couldn't be supported and I think the proc macro is being a little too conservative.

This strikes me as a regression because I thought this might have been possible at some point, but I can't recall exactly when it happened. I lack specific examples, but will update this issue as I find more specific information.

[jstarry]

@hgzimmerman this will no longer be supported for both components and tags when this change lands: #756 FYI

[hgzimmerman]

I see that this is fine given that oninput=self.link.callback(Some) should now work. Granted, you are typing more characters, but at least "tuple-functions" should work as intended in both vtags and components.
Close this upon completion of #756.

[jstarry]

Closed because #783 removes support for transforming closures into callbacks