Pull out libcrux::kem into a standalone crate

[jschneider-bensch]

This PR pulls out the KEM module from libcrux into a standalone crate in the workspace.

For now, I have changed visibility to pub of any pub (crate) items the module depended on in libcrux and libcrux-ml-kem. I have not yet re-exported the standalone KEM API in libcrux to avoid cyclic dependency, since the standalone crate still depends on the ecdh module in libcrux.

I've also changed the naming from Kyber to ML-KEM in the standalone crate.

[franziskuskiefer]

Thanks, generally looks good to me with a few nits.

[jschneider-bensch]

I'm not sure about the tests for libcrux-kem: I've moved some tests for ML-KEM from the top-level tests directory into the crate, in the process also removing some that were duplicated in libcrux-ml-kem. Those tests seem to actually test a re-exportet libcrux-ml-kem API though, so might be better placed in there? Other KEM related tests in the top-level seem to test hpke APIs.

[franziskuskiefer]

Thanks, lgtm with a few nits.

Can we keep a version of the implementation notes PDF from the old kyber folder?

[jschneider-bensch]

Can we keep a version of the implementation notes PDF from the old kyber folder?

I put a copy in libcrux-ml-kem (where the path was already excluded from packaging in Cargo.toml).