chore: VID disperse also return payload commitment

[ggutoski]

Description

closes: #369

• With this addition of the payload commitment to the return value of disperse, the return type would become a 3-tuple. This is unruly and clippy complained about type complexity when I tried it. (See fc4548c.) So instead I created a new struct VidDisperse to aggregate these values in a more legibile return type and give them names.
• I took this opportunity to shorten/clarify some names.
• Bizarre behaviour from the compiler forced me to add an unnecessary as cast:

  jellyfish/primitives/src/vid/advz.rs

  Lines 341 to 346 in ddea973

  	for poly_commit in common.poly_commits.iter() {
  	// TODO compiler bug? `as` should not be needed here!
  	(poly_commit as &P::Commitment)
  	.serialize_uncompressed(&mut hasher)
  	.map_err(vid)?;
  	}

---

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• 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