Should `PbList` and `PbMap` provide a `deepCopy` method?

I realized while refactoring a use site that `PbMap` and `PbList` don't provide a `deepCopy` method and users have to implement their own deep copy functions.

For `PbList`, we provide a constructor `PbList.from(List from)` for shallow copy, but deep copying needs to be implemented by the user.

For `PbMap`, we don't provide a shallow constructor or a `deepCopy` method.

I think it might make sense to add:

- `PbMap.deepCopy`, `PbList.deepCopy`
- `PbMap` constructor for shallow copying, for consistency with `PbList`