Extract minimal Rialto-Sub and Millau-Sub clients (#365)

* extracting sub clients

* fmt + lost docs

* revert enum BridgeInstance

* apply suggestions from review

* explicite debug impl

* remove unused imports from Millau

* fix typo

* fix instance + API name

* Update relays/ethereum/src/ethereum_sync_loop.rs

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

* separate crates for millau and rialto client

* cargo fmt

* fix

* fmt

* remove no_std support

* fix compilation again

* Update relays/substrate-client/Cargo.toml

* Update relay clients to Substrate 2.0

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

bridges/relays/substrate-client/src/lib.rs

@@ -0,0 +1,46 @@
+// Copyright 2019-2020 Parity Technologies (UK) Ltd.
+// This file is part of Parity Bridges Common.
+
+// Parity Bridges Common is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Parity Bridges Common is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Parity Bridges Common.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Tools to interact with (Open) Ethereum node using RPC methods.
+
+#![warn(missing_docs)]
+
+mod chain;
+mod client;
+mod error;
+mod rpc;
+
+pub use crate::chain::{Chain, TransactionSignScheme};
+pub use crate::client::{Client, OpaqueGrandpaAuthoritiesSet};
+pub use crate::error::{Error, Result};
+
+/// Substrate connection params.
+#[derive(Debug, Clone)]
+pub struct ConnectionParams {
+	/// Substrate RPC host.
+	pub host: String,
+	/// Substrate RPC port.
+	pub port: u16,
+}
+
+impl Default for ConnectionParams {
+	fn default() -> Self {
+		ConnectionParams {
+			host: "localhost".into(),
+			port: 9933,
+		}
+	}
+}