storage doublemap in decl_storage (#1918)

* factorization

* introduce GenericUnhashedStorage

* implement generator and storage

* impl double map in storage macro

* improve StorageDoubleMapXX methods

* remove storage from example and impl test

* remove old comments

* wasm compatible

* improve imports

* rename storages

* update runtime impl version

* make code less verbose

* impl hash config for second key in double map

hash available are all of Hashable trait

* use double map in decl_storage for contract

* fix double map config issue

* add hasher into metadata

* update impl version and build wasm

* doc

* add attrs

* update metadata version

* update runtime version

* fix unused storage

substrate/srml/metadata/src/lib.rs

@@ -263,6 +263,12 @@ pub enum StorageFunctionType {
 		value: DecodeDifferentStr,
 		is_linked: bool,
 	},
+	DoubleMap {
+		key1: DecodeDifferentStr,
+		key2: DecodeDifferentStr,
+		value: DecodeDifferentStr,
+		key2_hasher: DecodeDifferentStr,
+	},
 }
 
 /// A storage function modifier.
@@ -304,8 +310,10 @@ pub enum RuntimeMetadata {
 	V0(RuntimeMetadataDeprecated),
 	/// Version 1 for runtime metadata. No longer used.
 	V1(RuntimeMetadataDeprecated),
-	/// Version 2 for runtime metadata.
-	V2(RuntimeMetadataV2),
+	/// Version 2 for runtime metadata. No longer used.
+	V2(RuntimeMetadataDeprecated),
+	/// Version 3 for runtime metadata.
+	V3(RuntimeMetadataV3),
 }
 
 /// Enum that should fail.
@@ -325,10 +333,10 @@ impl Decode for RuntimeMetadataDeprecated {
 	}
 }
 
-/// The metadata of a runtime version 2.
+/// The metadata of a runtime.
 #[derive(Eq, Encode, PartialEq)]
 #[cfg_attr(feature = "std", derive(Decode, Debug, Serialize))]
-pub struct RuntimeMetadataV2 {
+pub struct RuntimeMetadataV3 {
 	pub modules: DecodeDifferentArray<ModuleMetadata>,
 }