Use persistent list length methods

In `./lean_client/containers/src/state.rs` there is such line:
```
// Count validators by iterating (since PersistentList doesn't have len())
```

But that's not true? PersistentList has `len_usize()` method: https://github.com/grandinetech/grandine/blob/5bdc78763c8959ad689c79d51d7d59978460bb1e/ssz/src/persistent_list.rs#L293.

So if there are more such places, they all need to be changed.