Codegen assumes presence of frame_system::AccountData

[ascjones]

Currently the AccountData storage type is hardcoded:

subxt/codegen/src/api/mod.rs

Lines 228 to 237 in ca5345c

	pub type DefaultAccountData = self::system::storage::Account;
	impl ::subxt::AccountData<::subxt::DefaultConfig> for DefaultAccountData {
	fn nonce(result: &<Self as ::subxt::StorageEntry>::Value) -> <::subxt::DefaultConfig as ::subxt::Config>::Index {
	result.nonce
	}
	fn storage_entry(account_id: <::subxt::DefaultConfig as ::subxt::Config>::AccountId) -> Self {
	Self(account_id)
	}
	}

which is required for fetching the nonce for constructing the Extra data for an Extrinsic. It assumes the presence of the system pallet and that the AccountData type has a nonce field. So we should consider whether this should be made configurable, or at least detect if the system pallet is not present with that Storage entry. Currently we are assuming it is always there. https://github.com/paritytech/subxt

From #309

[Xanewok]

I understand that #409 fixed this; let's reopen this it's still relevant.