New API for KEMs

[praveksharma]

A lot of KEM usecases (such as TLS), allow preserving state between keygen and decaps; this means that the matrix A can be kept expanded and makes input validation for the secret key unnecessary. This would make KEM operations a lot faster but also raises the question of what the new KEM API should look like. The consensus, when this topic came up during a status call, was that the API should allow OQS user to interoperate as much as possible between the different existing standards.

Here are the proposed new functions for mlkem-native (from PQCP) which shall operate on internal data structures:

crypto_kem_keypair_struct
crypto_kem_keypair_derand_struct
crypto_kem_marshal_pk
crypto_kem_marshal_sk

crypto_kem_parse_pk
crypto_kem_encaps_struct
crypto_kem_encaps_derand_struct

crypto_kem_parse_sk
crypto_kem_decaps_struct

The old API would then be wrappers around these new functions:

crypto_kem_keypair: crypto_kem_keypair_struct + crypto_kem_marshal_pk + crypto_kem_marshal_sk
crypto_kem_encaps: crypto_kem_parse_pk + crypto_kem_encaps_struct
crypto_kem_decaps: crypto_kem_parse_sk + crypto_kem_decaps_struct

Along with these new functions to aid interop:

crypto_kem_sk_from_seed
crypto_kem_pk_from_sk

Does this API miss any particular use case?