EnumerableStorageMap (#1763)

* Refactor decl_storage a bit to allow easier impl of linked map.

* A bunch of refactorings for storage generation.

- Rename StorageMap and ChildrenStorageMap to avoid confusion with generator::StorageMap.
- Separate implementation from the procedural macro code to clean it up.
- Make sure that genesis is initialised using the `StorageValue/StorageMap`
  generated implementations instead of going RAW.

* WiP: Writing test.

* Basic implementation.

* Implement enumeration.

* Fix non-std issues.

* fix warning

* Fix test-client.

* Address review grumbles - part 1

* Avoid cloning the key, relax Storage requirements.

* Rebuild runtime.

* Remove dangling todo.

substrate/srml/support/procedural/src/storage/mod.rs

@@ -24,6 +24,8 @@ use srml_support_procedural_tools::{ToTokens, Parse, custom_keyword, custom_keyw
 use syn::{Ident, Token};
 use syn::token::CustomKeyword;
 
+mod impls;
+
 pub mod transformation;
 
 /// Parsing usage only
@@ -116,6 +118,7 @@ struct DeclStorageBuild {
 #[derive(Parse, ToTokens, Debug)]
 enum DeclStorageType {
 	Map(DeclStorageMap),
+	LinkedMap(DeclStorageLinkedMap),
 	Simple(syn::Type),
 }
 
@@ -127,6 +130,14 @@ struct DeclStorageMap {
 	pub value: syn::Type,
 }
 
+#[derive(Parse, ToTokens, Debug)]
+struct DeclStorageLinkedMap {
+	pub map_keyword: ext::CustomToken<LinkedMapKeyword>,
+	pub key: syn::Type,
+	pub ass_keyword: Token![=>],
+	pub value: syn::Type,
+}
+
 #[derive(Parse, ToTokens, Debug)]
 struct DeclStorageDefault {
 	pub equal_token: Token![=],
@@ -141,3 +152,4 @@ custom_keyword_impl!(DeclStorageBuild, "build", "storage build config");
 custom_keyword_impl!(AddExtraGenesis, "add_extra_genesis", "storage extra genesis");
 custom_keyword_impl!(DeclStorageGetter, "get", "storage getter");
 custom_keyword!(MapKeyword, "map", "map as keyword");
+custom_keyword!(LinkedMapKeyword, "linked_map", "linked_map as keyword");