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/support/src/metadata.rs

@@ -16,7 +16,7 @@
 
 pub use srml_metadata::{
 	DecodeDifferent, FnEncode, RuntimeMetadata,
-	ModuleMetadata, RuntimeMetadataV2,
+	ModuleMetadata, RuntimeMetadataV3,
 	DefaultByteGetter, RuntimeMetadataPrefixed,
 };
 
@@ -36,8 +36,8 @@ macro_rules! impl_runtime_metadata {
 	) => {
 		impl $runtime {
 			pub fn metadata() -> $crate::metadata::RuntimeMetadataPrefixed {
-				$crate::metadata::RuntimeMetadata::V2 (
-					$crate::metadata::RuntimeMetadataV2 {
+				$crate::metadata::RuntimeMetadata::V3 (
+					$crate::metadata::RuntimeMetadataV3 {
 						modules: $crate::__runtime_modules_to_metadata!($runtime;; $( $rest )*),
 					}
 				).into()
@@ -377,8 +377,8 @@ mod tests {
 			event_module2::Module with Event Storage Call,
 	);
 
-	const EXPECTED_METADATA: RuntimeMetadata = RuntimeMetadata::V2(
-		RuntimeMetadataV2 {
+	const EXPECTED_METADATA: RuntimeMetadata = RuntimeMetadata::V3(
+		RuntimeMetadataV3 {
 		modules: DecodeDifferent::Encode(&[
 			ModuleMetadata {
 				name: DecodeDifferent::Encode("system"),