Storage map iter (#148)

* Add example file

* Fmt

* Add KeyIter.

* Add iter method to store proc-macro.

* Fetch all values at once.

* Update docs.

* Run rustfmt.

Co-authored-by: Andrew Jones <ascjones@gmail.com>

proc-macro/tests/balances.rs

@@ -89,8 +89,8 @@ subxt_test!({
     account: Alice,
     step: {
         state: {
-            alice: AccountStore { account_id: &alice },
-            bob: AccountStore { account_id: &bob },
+            alice: &AccountStore { account_id: &alice },
+            bob: &AccountStore { account_id: &bob },
         },
         call: TransferCall {
             to: &bob,