Fix for incorrect era when compounding

[Dinonard]

Pull Request Summary

Reproduced and fixed issue where incorrect contract era stake is overwritten.

Check list

• [ ] contains breaking changes
• [ ] adds new feature
• [ ] modifies existing feature (bug fix or improvements)
• [ ] relies on other tasks
• [ ] documentation changes
• [ ] tests and/or benchmarks are included
• [ ] changed API client type definition or chain metadata

This pull request makes the following changes:

[codespool]

@Dinonard I got the logic in, but I'm having trouble with the contract type and converting it to Vec<u8>, mainly because I couldn't find a method to get it in raw format from iter_prefix_from. (if I used the storage_map_final_key from ledger like you suggested, it complained there are encodeLike traits missing on SmartContract type, and if I added that trait, then the runtime build failed - i figured we don't want to touch runtime code for this)

I tried to use SmartContract type as key for BTreeMap so I don't have to convert, but that got me an overflow issue from the scale codec.

In the end I used I used PrefixIterator::last_raw_key but that got me into ownership issues.

I know there's probably a super simple solution, but I spent a good part of last night trying to find it, so any advice is welcome 🙏

[codespool]

@Dinonard thanks for the comments! Since this is resolved from another PR, I'll just fix them for the sake of learning and close the PR.

[akru]

@Dinonard should we close this PR?