Factor out each-block code into helper functions

[Swatinem]

This changes the generated code for EachBlocks into a bunch of helpers, as suggested here: #3504 (comment)

What still needs to be done is everything around animations/transitions and unkeyed updates, but even without it, it is ready now and passes all the current tests.

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Tests

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[Swatinem]

Still interested in seeing this through, though I didn’t really have the motivation recently to work on it.
Closing for now, will open a new PR if I have something fresh.