Sassafras Consensus Pallet (#1577)

This PR introduces the pallet for Sassafras consensus.

## Non Goals

The pallet delivers only the bare-bones and doesn't deliver support for
auxiliary functionalities such as equivocation report and support for
epoch change via session pallet.

These functionalities were drafted in the [main
PR](#1336), but IMO is
better to introduce this auxiliary stuff in a follow up PR and after
client code.

## Potential follow ups

#2364

---------

Co-authored-by: Sebastian Kunert <[email protected]>
Co-authored-by: Koute <[email protected]>
Co-authored-by: Michal Kucharczyk <[email protected]>
Co-authored-by: André Silva <[email protected]>
Co-authored-by: Bastian Köcher <[email protected]>

Cargo.toml

@@ -346,6 +346,7 @@ members = [
 	"substrate/frame/root-testing",
 	"substrate/frame/safe-mode",
 	"substrate/frame/salary",
+	"substrate/frame/sassafras",
 	"substrate/frame/scheduler",
 	"substrate/frame/scored-pool",
 	"substrate/frame/session",

substrate/frame/sassafras/Cargo.toml

@@ -0,0 +1,59 @@
+[package]
+name = "pallet-sassafras"
+version = "0.3.5-dev"
+authors = ["Parity Technologies <[email protected]>"]
+edition = "2021"
+license = "Apache-2.0"
+homepage = "https://substrate.io"
+repository = "https://github.com/paritytech/substrate/"
+description = "Consensus extension module for Sassafras consensus."
+readme = "README.md"
+publish = false
+
+[package.metadata.docs.rs]
+targets = ["x86_64-unknown-linux-gnu"]
+
+[dependencies]
+scale-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"] }
+frame-benchmarking = { path = "../benchmarking", default-features = false, optional = true  }
+frame-support = { path = "../support", default-features = false  }
+frame-system = { path = "../system", default-features = false  }
+log = { version = "0.4.17", default-features = false }
+sp-consensus-sassafras = { path = "../../primitives/consensus/sassafras", default-features = false, features = ["serde"] }
+sp-io = { path = "../../primitives/io", default-features = false  }
+sp-runtime = { path = "../../primitives/runtime", default-features = false }
+sp-std = { path = "../../primitives/std", default-features = false }
+
+[dev-dependencies]
+array-bytes = "6.1"
+sp-core = { path = "../../primitives/core" }
+
+[features]
+default = [ "std" ]
+std = [
+	"frame-benchmarking?/std",
+	"frame-support/std",
+	"frame-system/std",
+	"log/std",
+	"scale-codec/std",
+	"scale-info/std",
+	"sp-consensus-sassafras/std",
+	"sp-io/std",
+	"sp-runtime/std",
+	"sp-std/std",
+]
+runtime-benchmarks = [
+	"frame-benchmarking/runtime-benchmarks",
+	"frame-support/runtime-benchmarks",
+	"frame-system/runtime-benchmarks",
+	"sp-runtime/runtime-benchmarks",
+]
+try-runtime = [
+	"frame-support/try-runtime",
+	"frame-system/try-runtime",
+	"sp-runtime/try-runtime",
+]
+# Construct dummy ring context on genesis.
+# Mostly used for testing and development.
+construct-dummy-ring-context = []

substrate/frame/sassafras/README.md

@@ -0,0 +1,8 @@
+Runtime module for SASSAFRAS consensus.
+
+- Tracking issue: https://github.com/paritytech/polkadot-sdk/issues/41
+- Protocol RFC proposal: https://github.com/polkadot-fellows/RFCs/pull/26
+
+# ⚠️ WARNING ⚠️
+
+The crate interfaces and structures are experimental and may be subject to changes.