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/primitives/mixnet/Cargo.toml

@@ -0,0 +1,30 @@
+[package]
+description = "Substrate mixnet types and runtime interface"
+name = "sp-mixnet"
+version = "0.1.0-dev"
+license = "Apache-2.0"
+authors = ["Parity Technologies <admin@parity.io>"]
+edition = "2021"
+homepage = "https://substrate.io"
+repository = "https://github.com/paritytech/substrate/"
+readme = "README.md"
+
+[package.metadata.docs.rs]
+targets = ["x86_64-unknown-linux-gnu"]
+
+[dependencies]
+codec = { package = "parity-scale-codec", version = "3.6.1", default-features = false, features = ["derive"] }
+scale-info = { version = "2.5.0", default-features = false, features = ["derive"] }
+sp-api = { default-features = false, path = "../api" }
+sp-application-crypto = { default-features = false, path = "../application-crypto" }
+sp-std = { default-features = false, path = "../std" }
+
+[features]
+default = [ "std" ]
+std = [
+	"codec/std",
+	"scale-info/std",
+	"sp-api/std",
+	"sp-application-crypto/std",
+	"sp-std/std",
+]