Mixnet integration (#1346)

See #1345, <https://github.com/paritytech/substrate/pull/14207>.

This adds all the necessary mixnet components, and puts them together in
the "kitchen-sink" node/runtime. The components added are:

- A pallet (`frame/mixnet`). This is responsible for determining the
current mixnet session and phase, and the mixnodes to use in each
session. It provides a function that validators can call to register a
mixnode for the next session. The logic of this pallet is very similar
to that of the `im-online` pallet.
- A service (`client/mixnet`). This implements the core mixnet logic,
building on the `mixnet` crate. The service communicates with other
nodes using notifications sent over the "mixnet" protocol.
- An RPC interface. This currently only supports sending transactions
over the mixnet.

---------

Co-authored-by: David Emett <dave@sp4m.net>
Co-authored-by: Javier Viola <javier@parity.io>

substrate/bin/node/testing/src/genesis.rs

@@ -109,5 +109,6 @@ pub fn config_endowed(code: Option<&[u8]>, extra_endowed: Vec<AccountId>) -> Run
 			trash_data_count: Default::default(),
 			..Default::default()
 		},
+		mixnet: Default::default(),
 	}
 }

substrate/bin/node/testing/src/keyring.rs

@@ -64,6 +64,7 @@ pub fn to_session_keys(
 		babe: sr25519_keyring.to_owned().public().into(),
 		im_online: sr25519_keyring.to_owned().public().into(),
 		authority_discovery: sr25519_keyring.to_owned().public().into(),
+		mixnet: sr25519_keyring.to_owned().public().into(),
 	}
 }