kem: versioning

[tarcieri]

I'm not sure exactly what happened, and it's probably just a mistake on my part, but at some point the kem crate version jumped from 0.3.0-pre.2 to 0.4.0-pre.0. We're currently on v0.4.0-rc.5.

Seems like a bit of a mess. Should we go back to v0.3 since there's never been a stable release? I guess I could yank all of the v0.4.0 prereleases.

[tarcieri]

I went back to kem v0.3.0-rc.0 in #2225, released new versions of all of the KEM crates that depend on it in RustCrypto/KEMs#187, and yanked all prereleases other than v0.3.0-rc.0.

The next stable version will be v0.3.0.

[tarcieri]

Aah, now I see why the version was bumped to v0.4.0-pre*: ml-kem v0.2.1 is using kem with a ^0.3.0-pre.0 requirement, so I probably bumped kem so it wouldn't cause issues there.

Perhaps what I could do instead is release ml-kem v0.2.1 with a =0.3.0-pre.0 requirement.