Add Clone to KemCore and KemCore::EncapsulationKey (#79)

Add Clone to `KemCore` and `KemCore::EncapsulationKey` so that
downstream it is possible to make items using generic elements Clone as
well. For example doing something like the following requires not only
`KemCore::EncapsulationKey` to implement `Clone` but also `KemCore`
itself.

```rs
#[derive(Clone)]
struct HandshakeMaterials<K: KemCore> {
    // ...
    ek: <K as KemCore>::EncapsulationKey,
    // ...
}
```

Co-authored-by: jmwample <[email protected]>

ml-kem/src/kem.rs

@@ -216,6 +216,7 @@ where
 
 /// An implementation of overall ML-KEM functionality.  Generic over parameter sets, but then ties
 /// together all of the other related types and sizes.
+#[derive(Clone)]
 pub struct Kem<P>
 where
     P: KemParams,

ml-kem/src/lib.rs

@@ -111,7 +111,7 @@ pub trait EncapsulateDeterministic<EK, SS> {
 }
 
 /// A generic interface to a Key Encapsulation Method
-pub trait KemCore {
+pub trait KemCore: Clone {
     /// The size of a shared key generated by this KEM
     type SharedKeySize: ArraySize;
 
@@ -128,6 +128,7 @@ pub trait KemCore {
     #[cfg(not(feature = "deterministic"))]
     type EncapsulationKey: Encapsulate<Ciphertext<Self>, SharedKey<Self>>
         + EncodedSizeUser
+        + Clone
         + Debug
         + PartialEq;
 
@@ -136,6 +137,7 @@ pub trait KemCore {
     type EncapsulationKey: Encapsulate<Ciphertext<Self>, SharedKey<Self>>
         + EncapsulateDeterministic<Ciphertext<Self>, SharedKey<Self>>
         + EncodedSizeUser
+        + Clone
         + Debug
         + PartialEq;