Convert Callback to be an enum to support Fn and FnOnce.

[lukerandall]

Closes #1039

[lukerandall]

@jstarry Thanks for your advice. I had tripped wrapping the callaback in an Option, but without the RefCell I obviously couldn't get it to work. One resulting issue now is the build is failing due to clippy warning about the complexity of the type:

error: very complex type used. Consider factoring parts into `type` definitions
  --> yew/src/callback.rs:19:18
   |
19 |     CallbackOnce(Rc<RefCell<Option<Box<dyn FnOnce(IN)>>>>),
   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |

I thought I'd solicit some input on how you'd break this up, since it's mostly about what to name the type, and how much of the type to extract. I was thinking something like:

type OptionalCallback<IN> = RefCell<Option<Box<dyn FnOnce(IN)>>>

[jstarry]

Nice work!

How about this:

type CallbackOnce<IN> = RefCell<Option<Box<dyn FnOnce(IN)>>>;

Also can you please add this function: Scope::callback_once to make it easy for devs to create one time callbacks?

(ComponentLink is aliased to scope btw)

[jstarry]

Just one small thing. Btw really appreciate the comment cleanup and your attention to detail!

[lukerandall]

Thanks, I agree Callback::once looks a lot better! Now if only I could remember to run cargo fmt before pushing 🙈