storage: Decode keys of storage entry iterations

[lexnv]

Fetching a query of all systems' accounts storage().system().account_iter() returns a key value struct:

• keys contain ()
• key_bytes contains twox System twox AccountIter [remaining-key-of-the-account]
• value contains the account value

subxt/subxt/src/storage/storage_type.rs

Lines 313 to 323 in b076f4c

	/// A pair of keys and values together with all the bytes that make up the storage address.
	/// `keys` is `None` if non-concat hashers are used. In this case the keys could not be extracted back from the key_bytes.
	#[derive(Clone, Debug, Eq, PartialEq, Ord, PartialOrd)]
	pub struct StorageKeyValuePair<T: Address> {
	/// The bytes that make up the address of the storage entry.
	pub key_bytes: Vec<u8>,
	/// The keys that can be used to construct the address of this storage entry.
	pub keys: T::Keys,
	/// The value of the storage entry.
	pub value: T::Target,
	}

From the user's perspective, the keys field makes it a bit harder to reason about the final key we provide.
In this example, keys field does not return the account ID part of the key.

In codegen, we pass () as keys to the account_iter fn. However, we know that the keys field in this case is similar to the account fn:

:subxt::ext::subxt_core::storage::address::StaticStorageKey::new(
   ::subxt::ext::subxt_core::utils::AccountId32(pub [u8; 32])
)

Example to reproduce this:

#![allow(missing_docs)]

use std::str::FromStr;
use subxt::ext::codec::Decode;
use subxt::utils::{AccountId32, H256};
use subxt::{Config, OnlineClient, SubstrateConfig};

type ChainConfig = SubstrateConfig;

#[subxt::subxt(runtime_metadata_insecure_url = "wss://rococo-asset-hub-rpc.polkadot.io")]
pub mod asset_hub {}

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let client =
        OnlineClient::<ChainConfig>::from_url("wss://rococo-asset-hub-rpc.polkadot.io").await?;

    let storage = client.storage().at_latest().await.unwrap();

    // Build a storage query to iterate over account information.
    let storage_query = asset_hub::storage().system().account_iter();

    // Get back an iterator of results (here, we are fetching 10 items at
    // a time from the node, but we always iterate over one at a time).
    let mut results = storage.iter(storage_query.clone()).await?;
    while let Some(Ok(kv)) = results.next().await {
        println!("Decoded key(s): {:?}", kv.keys);
        println!("Decoded key_bytes: {:?}", hex::encode(kv.key_bytes));
        println!("Value: {:?}", kv.value);
    }

    Ok(())
}