Implement contract_getStorage RPC API (#3944)

substrate/srml/contracts/src/exec.rs

@@ -29,6 +29,7 @@ pub type CallOf<T> = <T as Trait>::Call;
 pub type MomentOf<T> = <<T as Trait>::Time as Time>::Moment;
 pub type SeedOf<T> = <T as system::Trait>::Hash;
 pub type BlockNumberOf<T> = <T as system::Trait>::BlockNumber;
+pub type StorageKey = [u8; 32];
 
 /// A type that represents a topic of an event. At the moment a hash is used.
 pub type TopicOf<T> = <T as system::Trait>::Hash;
@@ -84,8 +85,6 @@ macro_rules! try_or_exec_error {
 	}
 }
 
-pub type StorageKey = [u8; 32];
-
 /// An interface that provides access to the external environment in which the
 /// smart-contract is executed.
 ///