Add an RPC method for calling a contract. (#3563)

* Sketch

* Some work on docs.

* Doc improvements.

* More docs.

* Some more docs.

* Yet another comment.

* Bump impl_version.

* Accept the block hash

* Use NumberOrHex

* Update node/rpc/src/contracts.rs

Co-Authored-By: Tomasz Drwięga <tomusdrw@users.noreply.github.com>

* Move rpc/primitives

substrate/node/rpc/src/lib.rs

@@ -30,3 +30,11 @@
 #![warn(missing_docs)]
 
 pub mod accounts;
+pub mod contracts;
+
+mod constants {
+	/// A status code indicating an error happened while trying to call into the runtime.
+	///
+	/// This typically means that the runtime trapped.
+	pub const RUNTIME_ERROR: i64 = 1;
+}