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