Support for pointers

In apps, entities often have references to other entities. For example, a `User` might have a "friends" collection of other `User`s and a "supervisor" `User`. This concept is known as _foreign keys_ in databases or _pointers_ in low-level software.

Strangely, Hive only supports saving collections of pointers in the form of a `HiveList`, not saving a single pointer.
I'd really like a `HivePointer` type of some sort which points to another entity, possibly in another box. Maybe, the `HivePointer` could have a `.resolve()` method which resolves the reference.