`KEM` trait

[jschneider-bensch]

Should be implemented in

• libcrux-x25519
• libcrux-p256
• libcrux-ml-kem
  • 512
  • 768
  • 1024
• libcrux-kem (hybrids)
  • xwing
  • ?

[keks]

I removed the ECDH blanket impl and added to specific ones, because we don't have an ECDH trait yet.

[keks]

I made some progress with the implementations and added P256 and ML-KEM512.

I noticed that the way ML-KEM is implemented makes it difficult to make use of the existing mutable references to the destination memory.

I also am not sure how to properly do validation in ML-KEM. I found a function validate_private_key_only, but I didn't know how to call that as it required a const param K that couldn't be inferred and I didn't know what to use. In general, that implementation is lacking when it comes to validation.

Tomorrow I'll expand on the existing implementation and move it into a macro that I can call for the other ML-KEMs (and hopefully Kyber) as well. Then I'll look at the hybrids.