Remove Tendermint for GRANDPA

Updates to polkadot-v0.9.40, with a variety of dependency updates accordingly.
Substrate thankfully now uses k256 0.13, pathing the way for #256. We couldn't
upgrade to polkadot-v0.9.40 without this due to polkadot-v0.9.40 having
fundamental changes to syncing. While we could've updated tendermint, it's not
worth the continued development effort given its inability to work with
multiple validator sets.

Purges sc-tendermint. Keeps tendermint-machine for #163.

Closes #137, #148, #157, #171. #96 and #99 should be re-scoped/clarified. #134
and #159 also should be clarified. #169 is also no longer a priority since
we're only considering temporal deployments of tendermint. #170 also isn't
since we're looking at effectively sharded validator sets, so there should
be no singular large set needing high performance.

tendermint/Cargo.toml

@@ -0,0 +1,24 @@
+[package]
+name = "tendermint-machine"
+version = "0.2.0"
+description = "An implementation of the Tendermint state machine in Rust"
+license = "MIT"
+repository = "https://github.com/serai-dex/serai/tree/develop/tendermint"
+authors = ["Luke Parker <lukeparker5132@gmail.com>"]
+edition = "2021"
+
+[dependencies]
+async-trait = "0.1"
+thiserror = "1"
+
+log = "0.4"
+
+parity-scale-codec = { version = "3", features = ["derive"] }
+
+futures = "0.3"
+tokio = { version = "1", features = ["macros", "sync", "time", "rt"] }
+
+sp-runtime = { git = "https://github.com/serai-dex/substrate", version = "7.0.0", optional = true }
+
+[features]
+substrate = ["sp-runtime"]