StorageVec::insert() does not work with nested storage types

[ironcev]

E.g., having:

storage {
    nested_vec: StorageVec<StorageVec<u64>> = StorageVec {},
}

and inserting:

storage.nested_vec.insert(0, StorageVec {});
storage.nested_vec.insert(0, StorageVec {});

will revert at runtime at the second insert. At the same time:

storage.nested_vec.push(StorageVec {});
storage.nested_vec.push(StorageVec {});

works.

The same is with e.g. StorageVec<StorageString>.

For additional examples, see commented out tests in #6043 that are marked with TODOs that relate to this issue.

The bug might be in the fact that in the StorageVec::insert() these lines:

let write_offset = offset_calculator::<V>(count + 1);
let read_offset = offset_calculator::<V>(count);
write::<V>(key, write_offset, read::<V>(key, read_offset).unwrap());

will be calculating the same write_offset and read_offset for zero-size types.

To that, this issue is very likely an issue with all zero-sized types and not only storage types. It's just that nobody usually stores zero-size types in the storage.

As a general remark, I am not sure if fixing this issue makes sense before Storage v3.0 is implemented.