Extract RPC definitions from RPC crate. (#3502)

* Extract author API from the substrate-rpc crate.

* Split out API from RPC.

* Clean up naming.

* Fix tests.

* Shorten error translations.

* Update Cargo.lock

substrate/core/rpc/api/src/helpers.rs

@@ -0,0 +1,31 @@
+// Copyright 2018-2019 Parity Technologies (UK) Ltd.
+// This file is part of Substrate.
+
+// Substrate 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.
+
+// Substrate 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 Substrate.  If not, see <http://www.gnu.org/licenses/>.
+
+use jsonrpc_core::futures::prelude::*;
+use futures03::{channel::oneshot, compat::Compat};
+
+/// Wraps around `oneshot::Receiver` and adjusts the error type to produce an internal error if the
+/// sender gets dropped.
+pub struct Receiver<T>(pub Compat<oneshot::Receiver<T>>);
+
+impl<T> Future for Receiver<T> {
+	type Item = T;
+	type Error = jsonrpc_core::Error;
+
+	fn poll(&mut self) -> Poll<Self::Item, Self::Error> {
+		self.0.poll().map_err(|_| jsonrpc_core::Error::internal_error())
+	}
+}