Store fields and field instances separately

This is what Features did for the 2.x branch: #1064472: separate fields from field instances

Quoting that issue:

Fields exports in features combine in a single export what are really distinct forms of data: the base field definition and the field instance information. Therefore, field definitions are duplicated in multiple places, one per field instance export. This duplication produces several difficulties including:

To make a change to the field definition, it is necessary to update multiple field exports rather than just one.

Even though this module doesn't have as big of a problem with this as features, due to the different way it is meant to be used, storing fields and instances separately would still allow for a smoother flow, better exports and better diffs.