Stop using validity bitmaps in Rust union serialization

[Wumpf]

What

Simplifies the rust serialization of unions. Disallowed the use of nullable union fields in the process.
Unfortunately this makes the codegen overall more complicated, not less. There's definitely some low hanging fruit to turn this around, but it eluded my grasp - every attempt to formulate more general rules got stuck on some other failure case.

Run cargo test -p re_types and pixi run python ./tests/roundtrips.py.

• Run main tests

(branch name is a misnomer, too late to change)

Checklist

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To run all checks from main, comment on the PR with @rerun-bot full-check.

[Wumpf]

@rerun-bot full-check

[github-actions]

Started a full build: https://github.com/rerun-io/rerun/actions/runs/8971221018

[Wumpf]

passed main tests, phew

[Wumpf]

@rerun-bot full-check

[github-actions]

Started a full build: https://github.com/rerun-io/rerun/actions/runs/8974934710

[emilk]

Great!