In this PR, the Database interface in `core/state` has been extended
with one more function:
```go
// Iteratee returns a state iteratee associated with the specified state root,
// through which the account iterator and storage iterator can be created.
Iteratee(root common.Hash) (Iteratee, error)
```
With this additional abstraction layer, the implementation details can be hidden
behind the interface. For example, state traversal can now operate directly on
the flat state for Verkle or binary trees, which do not natively support traversal.
Moreover, state dumping will now prefer using the flat state iterator as
the primary option, offering better efficiency.
Edit: this PR also fixes a tiny issue in the state dump, marshalling the
next field in the correct way.
0ba4314321