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/core/rpc/src/chain/mod.rs

@@ -37,6 +37,7 @@ use client::{
 };
 use jsonrpc_pubsub::{typed::Subscriber, SubscriptionId};
 use primitives::{H256, Blake2Hasher};
+use rpc_primitives::number;
 use sr_primitives::{
 	generic::{BlockId, SignedBlock},
 	traits::{Block as BlockT, Header, NumberFor},