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/lib.rs

@@ -0,0 +1,34 @@
+// Copyright 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/>.
+
+//! Substrate RPC interfaces.
+//!
+//! A collection of RPC methods and subscriptions supported by all substrate clients.
+
+#![warn(missing_docs)]
+
+mod errors;
+mod helpers;
+mod subscriptions;
+
+pub use jsonrpc_core::IoHandlerExtension as RpcExtension;
+pub use subscriptions::Subscriptions;
+pub use helpers::Receiver;
+
+pub mod author;
+pub mod chain;
+pub mod state;
+pub mod system;