Fix RERUN_FLUSH_NUM_BYTES and data size estimations

[emilk]

What

• Closes DataRow.total_size_bytes is wrong, so RERUN_FLUSH_NUM_BYTES doesn't work, leading to huge messages in our example .rrds #5078

The first commit is the actual fix: the size calculations were completely wrong for fixed-size array and SmallVec, leading the batcher to severely under-counting how many bytes it had processed, leading it to effectively ignore RERUN_FLUSH_NUM_BYTES , which in turn lead to example .rrds which big batches of data, which streams poorly.

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I have tested the web demo (if applicable):
  • Using newly built examples: app.rerun.io
  • Using examples from latest main build: app.rerun.io
  • Using full set of examples from nightly build: app.rerun.io
• The PR title and labels are set such as to maximize their usefulness for the next release's CHANGELOG
• If applicable, add a new check to the release checklist!

• PR Build Summary
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• Recent benchmark results
• Wasm size tracking

[github-actions]

Size changes

Name	main	5086/merge	Change
dna.rrd	552.96 kiB	655.36 kiB	+18.52%
plots.rrd	1054.72 kiB	163.84 kiB	-84.47%
structure_from_motion.rrd	6.51 MiB	6.84 MiB	+5.07%

[emilk]

⬆ this means it is working!