Vastly improved support for deserialized iteration

[teh-cmc]

Whether we like it or not, sometimes we've got to deserialize.

Up until now, the new APIs only provided a nice UX for the deserialization-free paths, while the deserialized path was particularly painful to use.

The source of the pain is, as so often, the lack of lending iterators in the stdlib. This PR fixes that.

Essentially, this:

let all_fill_mode_chunks = results.get_optional_chunks(&FillMode::name());
let all_fill_modes = results.iter_as(timeline, FillMode::name());
let mut all_fill_mode_iters = all_fill_mode_chunks
    .iter()
    .map(|chunk| chunk.iter_component::<FillMode>())
    .collect_vec();
let mut all_fill_modes_indexed = {
    let all_fill_modes =
        all_fill_mode_iters.iter_mut().flat_map(|it| it.into_iter());
    let all_fill_modes_indices = all_fill_mode_chunks.iter().flat_map(|chunk| {
        chunk.iter_component_indices(&timeline, &FillMode::name())
    });
    itertools::izip!(all_fill_modes_indices, all_fill_modes)
};

becomes this:

let all_fill_modes = results.iter_as(timeline, FillMode::name()).component::<FillMode>();

• DNM: requires Chunkified, deserialization-free visualizers for all standard shapes #7020

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[Wumpf]

ah nasty indeed how an arc is needed here. Kinda magic how that solves the chunk iterators