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/error.rs

@@ -0,0 +1,55 @@
+// 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/>.
+
+//! Substrate node RPC errors.
+
+use jsonrpsee::client::RequestError;
+use relay_utils::MaybeConnectionError;
+
+/// Result type used by Substrate client.
+pub type Result<T> = std::result::Result<T, Error>;
+
+/// Errors that can occur only when interacting with
+/// a Substrate node through RPC.
+#[derive(Debug)]
+pub enum Error {
+	/// An error that can occur when making an HTTP request to
+	/// an JSON-RPC client.
+	Request(RequestError),
+	/// The response from the client could not be SCALE decoded.
+	ResponseParseFailed(codec::Error),
+}
+
+impl From<RequestError> for Error {
+	fn from(error: RequestError) -> Self {
+		Error::Request(error)
+	}
+}
+
+impl MaybeConnectionError for Error {
+	fn is_connection_error(&self) -> bool {
+		matches!(*self, Error::Request(RequestError::TransportError(_)))
+	}
+}
+
+impl ToString for Error {
+	fn to_string(&self) -> String {
+		match self {
+			Self::Request(e) => e.to_string(),
+			Self::ResponseParseFailed(e) => e.what().to_string(),
+		}
+	}
+}