Option<T> read-like traits & deprecate MaybeSignal/MaybeProp

[zakstucke]

Got Option<T> working to automatically implement read-like traits for any T implementing read-like traits, so MaybeProp but generic for all RwSignal, Memo, Signal... etc.

^😍

I think my changes across the last few PR's now meet the requirements to deprecate MaybeSignal and MaybeProp but that's up to you.

This PR depends on #3076, so this shouldn't be merged until after that's finished with and merged.

The only actual change in this PR is the new trait_options.rs file, everything else is changes from the unmerged #3076 or deprecation stuff. Edit: and some more From<> impls for Signal.

I could implement Write-like traits too, wouldn't be hard, each op would just be a noop when None, but maybe this is too weird.

[gbj]

So, this is objectively nice, and a good idea to implement the traits on Option<T>.

I'd note that it's different from MaybeProp, which is more like Option<Signal<Option<T>>> but flattens them into a single Option<T> when you read it, whereas using the method in this PR yields an Option<Option<T>>.

I'd like to review in a little more depth. It also seems there is some overlapping code between this and the other PR, which led to a merge issue after I merged the other one, and also some duplicate code to review... Would you mind fixing the merge issues? Then I'll take a look again.

[zakstucke]

Fixed conflicts!

You're right I was forgetting MaybeProp equated to Option<Signal<Option<T>>>, I've made some updates to provide feature parity with MaybeProp:

• More From<> impls
• An OptProp (plz rename haha) type alias for Option<Signal<Option<T>>>
• A flatten() trait helper method for Option<&Option<T>>

Aside from the non StoredValue static T case, which I think is a rare enough usecase it could be defined in userland, after #3105 there should be complete feature parity with just using basic signals but the added benefit of:

• Native setting to dom nodes e.g. see impl IntoClass for Option<impl IntoClass> #3104
• Much improved From<> type inference, everything just comes from Signal automatically
• Less burden inside leptos to manage all of the impls needed for MaybeSignal/MaybeProp to be as dynamic as Signal

[zakstucke]

But to be clear I have no issue getting rid of the deprecations by the way!

As you've seen over the last few PR's I've done, my main goal is to "elevate" top-level Option<>s to a point where MaybeSignal/MaybeProp are never needed, and ideally not recommended to component libraries anymore.