This will make it less painful to add additional properties, which
should persist across writes, such as `name`.
Conflicts:
activerecord/lib/active_record/attribute_set.rb
There's a lot more that can be moved to these, but this felt like a good
place to introduce the object. Plans are:
- Remove all knowledge of type casting from the columns, beyond a
reference to the cast_type
- Move type_cast_for_database to these objects
- Potentially make them mutable, introduce a state machine, and have
dirty checking handled here as well
- Move `attribute`, `decorate_attribute`, and anything else that
modifies types to mess with this object, not the columns hash
- Introduce a collection object to manage these, reduce allocations, and
not require serializing the types