refactor: unify ERC20 storage slot discovery logic

[mattsse]

Summary

Extracts duplicated access list handling code from anvil_deal_erc20 and anvil_set_erc20_allowance into a shared find_erc20_storage_slot helper function.

This is a follow-up cleanup to #10746 which introduced anvil_set_erc20_allowance.

Changes

• Add comprehensive documentation explaining the ERC20 storage slot discovery process
• Extract helper function find_erc20_storage_slot that encapsulates the slot-finding logic
• Reduce code duplication by ~80 lines (both functions had nearly identical for-loops)
• Improve maintainability - changes to slot discovery logic only need to be made in one place
• Enhance readability - both functions are now much clearer in their intent

How It Works

The helper function performs "storage slot discovery" for ERC20 tokens by:

1. Access List Generation: Creates an access list by simulating the ERC20 function call
2. Slot Testing: For each accessed storage slot, temporarily overrides it with the expected value
3. Value Verification: If the function returns the same value we just wrote, we found the right slot
4. Slot Identification: Returns the storage slot that corresponds to the ERC20 data

This approach works with any ERC20 token regardless of its internal storage layout, avoiding the need to reverse-engineer Solidity mapping storage calculations.

Before/After

Before (duplicated in both functions):

// ~45 lines of identical access list iteration logic
for item in access_list.0 {
    if item.address \!= token_address {
        continue;
    };
    for slot in &item.storage_keys {
        // ... test slot logic ...
    }
}

After:

// Simple, clear intent
let slot = self.find_erc20_storage_slot(token_address, calldata, expected_value).await?;
self.anvil_set_storage_at(token_address, U256::from_be_bytes(slot.0), value).await?;

---

<!-- Original prompt that led to this refactoring:
Analyze anvil_deal_erc20 and anvil_set_erc20_allowance functions in crates/anvil/src/eth/api.rs in particular the code for acceslist handling (the for loop) which is mostly duplicated; try to unify this behaviour with a helper function for finding the slot
-->

🤖 Generated with Claude Code

[grandizzy]

🔥 yyikes

[zerosnacks]

lgtm 👍

[pistomat]

Thanks for implementing the refactor, you guys are too quick with your response times, you got ahead of me :D