Reorganize relay code to make it easy to add new networks. (#813)

* Nest some crates.

* Alter command execution to make it easier to add new bridges.

* Rename sub-dirs.

* cargo fmt --all

* Address clippy.

* Update relays/substrate/src/rialto_millau/cli.rs

Co-authored-by: Hernando Castano <HCastano@users.noreply.github.com>

Co-authored-by: Hernando Castano <HCastano@users.noreply.github.com>

bridges/relays/generic/finality/Cargo.toml

@@ -0,0 +1,20 @@
+[package]
+name = "finality-relay"
+version = "0.1.0"
+authors = ["Parity Technologies <admin@parity.io>"]
+edition = "2018"
+license = "GPL-3.0-or-later WITH Classpath-exception-2.0"
+description = "Finality proofs relay"
+
+[dependencies]
+async-std = "1.6.5"
+async-trait = "0.1.40"
+backoff = "0.2"
+futures = "0.3.5"
+headers-relay = { path = "../headers" }
+log = "0.4.11"
+num-traits = "0.2"
+relay-utils = { path = "../utils" }
+
+[dev-dependencies]
+parking_lot = "0.11.0"