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/example/src/lib.rs

@@ -66,7 +66,6 @@ decl_storage! {
 		// A map that has enumerable entries.
 		Bar get(bar) config(): linked_map T::AccountId => T::Balance;
 
-
 		// this one uses the default, we'll demonstrate the usage of 'mutate' API.
 		Foo get(foo) config(): T::Balance;
 	}