Skip MlKemParams

[kornelski]

MlKemParams is an enum, which means it's fully public, and can't have any private fields to store any other params in the future. These two data-less variants it's all it can have.

MlKem doesn't have its own state other than the params. Given that the params are trivial and non-extensible, this makes MlKem::new(params) pretend to be more than it can be. It's not constructing any object, it's just indirection for the params (calling ffi::init() is handled by keys' functions anyway).

So I've made MlKemParams and MlKem the same thing. mlkem::MlKem::MlKem768 is repetitive, so I've renamed MlKem(Params) to Algorithm. It can be used as mlkem::Algorithm where the context is needed.