Refactor apply_delta for Storage Maps

[Dominik1999]

Closes #751

Refactoring apply_delta for StorageSlots and StorageMaps

• AccountStorageDelta.cleared_items and AccountStorageDelta.updated_items only contain updates to value slots.
• AccountStorageDelta.updated_maps only contains updates to storage maps.

1. This PR introduces a new condition for on_account_storage_after_set_item. If the new value is to be applied to a StorageSlot whose index already has a maps_update, the value is NOT added as slot_update.

   That means the roots of StorageMaps do not end up in cleared_items or updated_items, because

   • in the kernel, the procedure fails if the index doesn't point to a slot of type map
   • in the kernel, maps update events happen in MASM before slot updates
   • there cannot be a new root without a change in the tree (~ entry in maps_update)

   Note: We need to ignore the StorageSlot update when it is triggered by a map update in the Host, because we cannot validate it. At this stage (when the event is being processed in the Host) we don't have access to the whole storage map.

2. There is also a set_map_item and get_map_item in AccountStorage now replicating the MASM procedures.

[bobbinth]

Looks good! Thank you! I left some comments inline. The main one about potentially enforcing storage layout restrictions in MASM.

[bobbinth]

What's the reason for returning both the old root and the old value? Could we get away with returning just the old value? If so, this could look something like this:

let old_value = storage_map.insert(key.into(), value);

// update the root of the storage map in the corresponding storage slot
let index = LeafIndex::new(index.into()).expect("index is u8 - index within range");
self.slots.insert(index, storage_map.root().into());

Ok(old_value)

[Dominik1999]

We return both in the MASM procedure and we wanted to have set_map_item here as close to the MASM implementation as possible.

#! Sets a map item in the account storage. Panics if 
#! - the index for the map is out of bounds, means >255
#! - the slot item at index is not a map
#!
#! Stack: [index, KEY, VALUE]
#! Output: [OLD_MAP_ROOT, OLD_MAP_VALUE, 0]

but we can also change this in MASM?

[bobbinth]

If we bring back StorageMap::apply_delta() as mentioned, this could look something like:

let new_root = self.maps.get_mut(&index)
    .ok_or(AccountError::StorageMapNotFound(index))?
    .apply_delta(map_delta);
let index = LeafIndex::new(slot_idx.into()).expect("index is u8 - index within range");
self.slots.insert(index, new_root);

[Dominik1999]

Which means we don't need AccountStorage::set_map_item anymore, right?

Separately, we should also implement AccountStorage::set_map_item() and AccountStorage::get_map_item() methods to mimic the ones in the transaction kernel.

Originally, we wanted to have those methods. Or we just expose those methods to the user and client and don't need them to use ourselves.

[Dominik1999]

@bobbinth I refactored the MASM code quite a bit. Now we have a new procedure, set_item_raw, that doesn't trigger any events.

[bobbinth]

Looks good! Thank you! I left some comments inline. Most of them are small and we can merge after they are addressed. There is one bigger comment, but it could be done in a future PR.

Also, let's update the changelog.