Reserve ability to create Pod to ViewCtx

[PoignardAzur]

This is another intermediary PR for the "restrict widget creation to Mutate pass" feature.

The basic idea is that, in the near future, it will be impossible to create a WidgetPod without a handle to the arena. In my current WIP code, that handle is passed through ViewCtx. Therefore, this PR changes all the sites in xilem that create a Pod and has them use a ViewCtx method instead.

I've tested most xilem examples and they all worked (except for variable_clock, which currently panics in the last main commit).

[PoignardAzur]

Note for reviewers: the most interesting changes are in:

• xilem_core/tests/one_of.rs
• xilem_core/src/element.rs
• xilem/src/lib.rs

Everything else is plumbing those API changes into method implementations.

[Philipp-M]

Oh, well, I wanted to start a (zulip-)topic already about having the ViewCtx "everywhere", good to see that we're converging on similar ideas I guess.
I have started something very similar here already (I haven't access to my PC where I've implemented it, I forgot to push it, I'll link to it when I have access later), it's basically about:

Have access to ViewCtx everywhere where the element is forwarded/created in Xilem (that means in SuperElement as done here, and as well in ElementSplice which I think will be useful for keyed viewsequences as well as more efficient tree updates in xilem_web)

[Philipp-M]

So this is my branch, looks quite similar, different ordering of the parameters, and access to the ViewCtx in SuperElement::with_downcast_val, I guess that can be merged somewhat.
But since your PR is more targeted, probably yours first, I'll do that afterwards.
I think generally it makes sense to also have access to the context in SuperElement::with_downcast_val, but it introduces a little bit complexity, though I expect this to be useful (I'm experimenting with this currently in xilem_web).

except for variable_clock, which currently panics in the last main commit

I think you should open an issue then, it works for me (on this branch as well as main).

Otherwise looks good to me.

[PoignardAzur]

I think you should open an issue then, it works for me (on this branch as well as main).

It's fixed now.