Move auxiliary types outside `crates/storage/src/store.rs`

[MegaRedHand]

The file is already ~2000 lines long. We should move auxiliary types and functions to other modules. Splitting the Store impl block into multiple modules is also on the table.

[MegaRedHand]

Welcome to ethlambda! Your plan sounds good. Feel free to open a PR.
Just a heads-up, but we have a CONTRIBUTING.md with useful information for contributors 😁

[MegaRedHand]

For anyone taking this, I suggest splitting the work in multiple PRs

[perfogic]

@MegaRedHand is this issue still valid, can i pick this ?

[MegaRedHand]

Yes! I'll assign it to you