ml-kem library friction

[jmwample]

PkeParams and KemParams are not public which makes it impossible to do anything useful (i.e. add implementations) for the key types ml_kem::kem::{EncapsulationKey, DecapsulationKey}

i.e. It is extremely unwieldy to do something like:

impl<P> Decapsulate<KEncodedCiphertext<P>, SharedKey<P>> for ml_kem::kem::DecapsulationKey<P>
where
    P: KemCore + EncodingSize + ParameterSet,
...

[jmwample]

Added a PR with a simple fix that allows KemParams to stay private while allowing (De|En)capsulationKeys to be used with generic parameters.

RustCrypto/KEMs#81