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/node/runtime/src/lib.rs

@@ -59,7 +59,7 @@ pub const VERSION: RuntimeVersion = RuntimeVersion {
 	impl_name: create_runtime_str!("substrate-node"),
 	authoring_version: 10,
 	spec_version: 91,
-	impl_version: 92,
+	impl_version: 93,
 	apis: RUNTIME_API_VERSIONS,
 };