Release `prio` 0.18 for draft-irtf-cfrg-vdaf-18

[tgeoghegan]

We should take whatever conformity changes are left, and whatever breaking changes we still want, and release prio 0.18.0 with draft-irtf-cfrg-vdaf-18 support.

n.b. it's a coincidence that the crate version and draft version line up. This is not and has never been guaranteed.

[tgeoghegan]

#1403 is related but not a blocker: since mod dp is experimental, we can make changes to its API in dot releases to 0.18 without breaking SemVer, and we can always promote it to public after releasing 0.18.0.

[tgeoghegan]

We released 0.18.1-alpha.3 yesterday. We'll keep cutting -alpha.x releases for now while we make sure that we've got the right bits in the API. But there's nothing else to track here.