Add signals pallet

Resolves #353

Implements code such that:

- 80% of validators (by stake) must be in favor of a signal for the network to
  be
- 80% of networks (by stake) must be in favor of a signal for it to be locked
  in
- After a signal has been locked in for two weeks, the network halts

The intention is to:

1) Not allow validators to unilaterally declare new consensus rules.

No method of declaring new consensus rules is provided by this pallet. Solely a
way to deprecate the current rules, with a signaled for successor. All nodes
must then individually decide whether or not to download and run a new node
which has new rules, and if so, which rules.

2) Not place blobs on chain.

Even if they'd be reproducible, it's just a lot of data to chuck on the
blockchain.

substrate/signals/pallet/Cargo.toml

@@ -0,0 +1,47 @@
+[package]
+name = "serai-signals-pallet"
+version = "0.1.0"
+description = "Signals pallet"
+license = "AGPL-3.0-only"
+repository = "https://github.com/serai-dex/serai/tree/develop/substrate/signals/pallet"
+authors = ["Luke Parker <lukeparker5132@gmail.com>"]
+edition = "2021"
+
+[package.metadata.docs.rs]
+all-features = true
+rustdoc-args = ["--cfg", "docsrs"]
+
+[dependencies]
+scale = { package = "parity-scale-codec", version = "3", default-features = false, features = ["derive"] }
+scale-info = { version = "2", default-features = false, features = ["derive"] }
+
+sp-core = { git = "https://github.com/serai-dex/substrate", default-features = false }
+sp-io = { git = "https://github.com/serai-dex/substrate", default-features = false }
+
+frame-system = { git = "https://github.com/serai-dex/substrate", default-features = false }
+frame-support = { git = "https://github.com/serai-dex/substrate", default-features = false }
+
+serai-primitives = { path = "../../primitives", default-features = false }
+validator-sets-pallet = { package = "serai-validator-sets-pallet", path = "../../validator-sets/pallet", default-features = false }
+
+[features]
+std = [
+  "scale/std",
+  "scale-info/std",
+
+  "sp-core/std",
+  "sp-io/std",
+
+  "frame-system/std",
+  "frame-support/std",
+
+  "serai-primitives/std",
+  "validator-sets-pallet/std",
+]
+
+runtime-benchmarks = [
+  "frame-system/runtime-benchmarks",
+  "frame-support/runtime-benchmarks",
+]
+
+default = ["std"]