the interface get_fields method should maybe return Field instances

[Jaymon]

Arguments for:

1. You're basically mapping the fields into a dict structure that mimics what the Field class is for
2. Using the Field and saying the interface method needs to return a list/dict of Field instances means all interfaces would automatically use the same interface, right now interfaces return dicts, but those dicts can have keys setup anyway they like.

Arguments against:

1. all interface methods return standard data structures that are converted into prom structures down the line (eg, the Query converts the stream or the list into an iterator)
2. The interface would need to import things from prom proper where traditionally the interface has been kept kind of separate though the interface gets passed prom proper interfaces like Schema. Query, and Config it doesn't actually import any of them.