REF: rename _data->_mgr
We have a long-term goal of reducing the exposed surface of the internals.  One difficulty is that it is not trivial to identify all the places that access the internal, in part because grepping for `._data` will turn up a ton of non-internals Index and EA attributes.  By renaming _data -> _mgr, we'll make it easier to identify places that we can de-internalize.

If this isn't a route we want to go down (in the past when I've pitched this it hasn't caught on), I'll try to keep this up-to-date so we an use the diff as the relevant reference.