Separate AKE from OPRF take 2 (#222)

* Separate AKE from OPRF

Introduce X25519 implementation

* Rename `AkeGroup` to `KeGroup` and `Group` to `OprfGroup`

* Add documentation to "Overview"

src/key_exchange/tripledh.rs

@@ -411,7 +411,7 @@ impl<G: Group, HashLen: ArrayLength<u8>> FromBytes for Ke2Message<G, HashLen> {
         )?;
 
         // Check the public key bytes
-        let server_e_pk = KeyPair::<CS::Group>::check_public_key(PublicKey::from_bytes(
+        let server_e_pk = KeyPair::<CS::OprfGroup>::check_public_key(PublicKey::from_bytes(
             &unchecked_server_e_pk[..key_len],
         )?)?;