Store `link.childtype, link.parenttype, or group.membertype` in `DataStore._type_attributes``.

[mylibrar]

    I think we should have some way to support getting `link.childtype, link.parenttype, or group.membertype` without converting `entry_type_` back to class.

Originally posted by @mylibrar in #900 (comment)

#900 (comment)

Potential solution: #900 (comment)