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/utils/chain-spec-builder/src/lib.rs

@@ -179,7 +179,7 @@ pub fn generate_authority_keys_and_store(
 				.map_err(|err| err.to_string())?
 				.into();
 
-		let (_, _, grandpa, babe, im_online, authority_discovery) =
+		let (_, _, grandpa, babe, im_online, authority_discovery, mixnet) =
 			chain_spec::authority_keys_from_seed(seed);
 
 		let insert_key = |key_type, public| {
@@ -198,6 +198,8 @@ pub fn generate_authority_keys_and_store(
 			sp_core::crypto::key_types::AUTHORITY_DISCOVERY,
 			authority_discovery.as_slice(),
 		)?;
+
+		insert_key(sp_core::crypto::key_types::MIXNET, mixnet.as_slice())?;
 	}
 
 	Ok(())