JSON-RPC client generation (#2778)

* Bump jsonrpc & generate clients.

* Initial version of JSON-RPC client.

* Re-wort

* Remove spurious `#[derive(Encode, Decode)]`

They did not compile, since `Encode` and `Decode` are deliberately not
implemented for `usize`.

Fixes #2742.

* Re-write rpc-client example.

* Update to jsonrpc=12.0.0

* Remove unnecessary import

* Bump version.

* Revert version bump.

* Bump again.

substrate/core/rpc-servers/src/lib.rs

@@ -89,9 +89,9 @@ pub fn start_ws(
 		.allowed_origins(map_cors(cors))
 		.start(addr)
 		.map_err(|err| match err {
-			ws::Error(ws::ErrorKind::Io(io), _) => io,
-			ws::Error(ws::ErrorKind::ConnectionClosed, _) => io::ErrorKind::BrokenPipe.into(),
-			ws::Error(e, _) => {
+			ws::Error::Io(io) => io,
+			ws::Error::ConnectionClosed => io::ErrorKind::BrokenPipe.into(),
+			e => {
 				error!("{}", e);
 				io::ErrorKind::Other.into()
 			}