chore!: new Advz constructor, Advz use u32 instead of usize

[ggutoski]

Description

closes: #463

---

• BREAKING API CHANGE: Advz::new no longer take multiplicity arg. Instead we choose multiplicity for the caller. (Currently we set mulitiplicity to 1, issue ADVZ intelligent choice of multiplicity #534 .) Also, reorder the arg list.
• new constructor Advz::with_multiplicity does what Advz::new used to do.
• remove limit on multiplicity as requested by @mrain
• rename payload_chunk_size -> recovery_threshold. Much clearer IMO.
• As discussed, convert everything that will be serialized in VID from usize to u32. Lots of ugly as usize everywhere, but at least these conversions are safe. Dammit, Rust.
• I'm unable to build locally with gpu-vid feature. @mrain we might need you to check the build with gpu-vid.

Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• Targeted PR against correct branch (main)
• Linked to GitHub issue with discussion and accepted design OR have an explanation in the PR that describes this work.
• Wrote unit tests
• Updated relevant documentation in the code
• Added a relevant changelog entry to the Pending section in CHANGELOG.md
• Re-reviewed Files changed in the GitHub PR explorer

[mrain]

LGTM