RPC Module for Relays (#80)

* Test defining an RPC API

* Add wrapper clients for the RPC API

* Implement most Ethereum RPCs

Does not include RPCs that require the bridge contract.

* Implement a few of the Substrate RPCs

Still missing proper error handling, as well as decoding responses from the
Substrate node.

* Make error handling more ergonomic

* Implement Substrate RPCs that use `state_call`

* Implement rest of Substrate RPCs

* Implement `eth_call` RPC

This can be used to implement higher level requests like fetching Substrate headers
from an Ethereum node.

* Build some of the higher level Ethereum RPCs

Uses the new Ethereum RPC interface to do so

* Build some of the higher level Substrate RPCs

* Remove old Ethereum RPC methods

* Remove old Substrate RPC methods

* Add some documentation to Substrate RPCs

* Fix typo in enum construction

* Revert commits `0f0435d` to `ca75502`

This range of commits was mainly trying to integrate the new RPC interface into the existing
codebase, however this turned out to be a little out of scope for the current PR. Instead this work
will be incorporated into a PR which aims to close #72.

* Add documentation to RPCs

* Rename functions in RPC API to conform to snake_case

* Check that header contains a number and hash

* Put doc comments on trait instead of impl methods

* Remove expect() calls

* Replace runtime API enums with consts

* Accept Bytes when submitting extrinsic

Let's us avoid using a runtime specific Extrinsic.

* Add strictly typed arguments to RPC API

Missing two methods right now, which require a `serde::Deserialize`
implemenation before they can be changed.

* Add `chain_getBlock` Substrate RPC

* Use typed arguments for `eth_estimateGas` and `eth_call`

* Silence dead code warnings

* Add check for logs bloom

* Remove unused variables

* Add documentation to RPC error enums

bridges/relays/ethereum/src/rpc_errors.rs

@@ -0,0 +1,98 @@
+// 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/>.
+
+#![allow(dead_code)]
+
+use jsonrpsee::raw::client::RawClientError;
+use jsonrpsee::transport::http::RequestError;
+use serde_json;
+
+type RpcHttpError = RawClientError<RequestError>;
+
+/// Contains common errors that can occur when
+/// interacting with a Substrate or Ethereum node
+/// through RPC.
+#[derive(Debug)]
+pub enum RpcError {
+	/// The arguments to the RPC method failed to serialize.
+	Serialization(serde_json::Error),
+	/// An error occured when interacting with an Ethereum node.
+	Ethereum(EthereumNodeError),
+	/// An error occured when interacting with a Substrate node.
+	Substrate(SubstrateNodeError),
+	/// An error that can occur when making an HTTP request to
+	/// an JSON-RPC client.
+	Request(RpcHttpError),
+	/// The response from the client could not be SCALE decoded.
+	Decoding(codec::Error),
+}
+
+impl From<serde_json::Error> for RpcError {
+	fn from(err: serde_json::Error) -> Self {
+		Self::Serialization(err)
+	}
+}
+
+impl From<EthereumNodeError> for RpcError {
+	fn from(err: EthereumNodeError) -> Self {
+		Self::Ethereum(err)
+	}
+}
+
+impl From<SubstrateNodeError> for RpcError {
+	fn from(err: SubstrateNodeError) -> Self {
+		Self::Substrate(err)
+	}
+}
+
+impl From<RpcHttpError> for RpcError {
+	fn from(err: RpcHttpError) -> Self {
+		Self::Request(err)
+	}
+}
+
+impl From<codec::Error> for RpcError {
+	fn from(err: codec::Error) -> Self {
+		Self::Decoding(err)
+	}
+}
+
+/// Errors that can occur only when interacting with
+/// an Ethereum node through RPC.
+#[derive(Debug)]
+pub enum EthereumNodeError {
+	/// Failed to parse response.
+	ResponseParseFailed(String),
+	/// We have received a header with missing fields.
+	IncompleteHeader,
+	/// We have received a receipt missing a `gas_used` field.
+	IncompleteReceipt,
+	/// An invalid Substrate block number was received from
+	/// an Ethereum node.
+	InvalidSubstrateBlockNumber,
+}
+
+/// Errors that can occur only when interacting with
+/// a Substrate node through RPC.
+#[derive(Debug)]
+pub enum SubstrateNodeError {
+	/// Request start failed.
+	StartRequestFailed(RequestError),
+	/// Error serializing request.
+	RequestSerialization(serde_json::Error),
+	/// Failed to parse response.
+	ResponseParseFailed,
+}