Instant sealing engine (#1767)

* add an instant sealing engine

* add sealing engine tests

* rename to InstantSeal

* update name in json

* whitespace

[ci:skip]

json/src/spec/engine.rs

@@ -24,6 +24,8 @@ use spec::BasicAuthority;
 pub enum Engine {
 	/// Null engine.
 	Null,
+	/// Instantly sealing engine.
+	InstantSeal,
 	/// Ethash engine.
 	Ethash(Ethash),
 	/// BasicAuthority engine.
@@ -44,6 +46,14 @@ mod tests {
 		let deserialized: Engine = serde_json::from_str(s).unwrap();
 		assert_eq!(Engine::Null, deserialized);
 
+		let s = r#"{
+			"InstantSeal": null
+		}"#;
+
+		let deserialized: Engine = serde_json::from_str(s).unwrap();
+		assert_eq!(Engine::InstantSeal, deserialized);
+
+
 		let s = r#"{
 			"Ethash": {
 				"params": {