srml-contracts: Refactor away some duplication in runtime functions. (#3257)

* srml-contracts: Storage access micro-optimization.

* srml-contracts: Refactor runtime functions to reduce duplication.

* Bump node runtime impl version.

substrate/node/runtime/src/lib.rs

@@ -81,7 +81,7 @@ pub const VERSION: RuntimeVersion = RuntimeVersion {
 	// implementation changes and behavior does not, then leave spec_version as
 	// is and increment impl_version.
 	spec_version: 135,
-	impl_version: 135,
+	impl_version: 136,
 	apis: RUNTIME_API_VERSIONS,
 };