Parameterize AccountData (#409)

* Add custom config example

* Make account data storage item configurable

* Fix contracts

* Regenerate polkadot codegen

* Fmt

* Specify different type for `MyConfig::Index`

* Update comment in custom config example

* Assign concrete types for default AccountData impl

* Fmt

* Fix contracts tests

* Fmt

* Add comments

* Unlink doc comment trait (subxt not in scope)

* Fix missing nonce field error message

* Update codegen/src/api/mod.rs

Co-authored-by: David <dvdplm@gmail.com>

* Update examples/custom_config.rs

Co-authored-by: David <dvdplm@gmail.com>

* Rename Nonce assoc type to Index for consistency

* Add module level docs about codegen assumptions

Co-authored-by: David <dvdplm@gmail.com>

codegen/src/api/calls.rs

@@ -116,7 +116,7 @@ pub fn generate_calls(
             where
                 T: ::subxt::Config,
                 X: ::subxt::SignedExtra<T>,
-                A: ::subxt::AccountData<T>,
+                A: ::subxt::AccountData,
             {
                 pub fn new(client: &'a ::subxt::Client<T>) -> Self {
                     Self { client, marker: ::core::marker::PhantomData }