Sealed trait to make (De|En)capsulationKeys more usable

[jmwample]

Make is possible to wrap kem::(De|En)capsulationKey in external packages without exposing KemParams publicly.

Instead of making KemParams public and allowing external types to implement the interface, I have added a public trait Params which has KemParams as a public supertrait whose name is not publicly exported (private module). Rust API Guidelines

This makes it possible to do things like:

struct WrappedDecapsulationKey<P: Params> {
    key: DecapsulationKey<P>
}

This is more convenient than working with <P as KemCore>::DecapsulationKey.

Closes #65

[tarcieri]

In the latest prereleases of ml-kem you should be able to use DecapsulationKey and EncapsulationKey being generic over the Decapsulate and Encapsulate traits.

Do those do what you need in a generic context? Ideally I would like to get rid of a lot of the bespoke traits that various crates in this repo have which don't interoperate with each other.

[jmwample]

I am going to close this PR and try your suggestion. The project has been on the back-burner for some time and when I circle back around I will likely rework around a number of other things.

Thank you for checking back in.

[tarcieri]

I realized there may be one gap still: RustCrypto/traits#2213