Implement tendermint_machine::Block for Substrate Blocks

Unfortunately, this immediately makes Tendermint machine capable of 
deployment as  crate since it uses a git reference. In the future, a 
Cargo.toml patch section for serai/substrate should be investigated. 
This is being done regardless as it's the quickest way forward and this 
is for Serai.

substrate/tendermint/Cargo.toml

@@ -12,3 +12,8 @@ parity-scale-codec = { version = "3.2", features = ["derive"] }
 
 async-trait = "0.1"
 tokio = { version = "1", features = ["macros", "sync", "time", "rt"] }
+
+sp-runtime = { git = "https://github.com/serai-dex/substrate", optional = true }
+
+[features]
+substrate = ["sp-runtime"]