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/client/mixnet/src/lib.rs

@@ -0,0 +1,44 @@
+// This file is part of Substrate.
+
+// Copyright (C) Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: GPL-3.0-or-later WITH Classpath-exception-2.0
+
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with this program. If not, see <https://www.gnu.org/licenses/>.
+
+//! Substrate mixnet service. This implements the [Substrate Mix Network
+//! Specification](https://paritytech.github.io/mixnet-spec/).
+
+#![warn(missing_docs)]
+#![forbid(unsafe_code)]
+
+mod api;
+mod config;
+mod error;
+mod extrinsic_queue;
+mod maybe_inf_delay;
+mod packet_dispatcher;
+mod peer_id;
+mod protocol;
+mod request;
+mod run;
+mod sync_with_runtime;
+
+pub use self::{
+	api::{Api, ApiBackend},
+	config::{Config, CoreConfig, SubstrateConfig},
+	error::{Error, RemoteErr},
+	protocol::{peers_set_config, protocol_name},
+	run::run,
+};
+pub use mixnet::core::{KxSecret, PostErr, TopologyErr};