Bump Rummager to 1.0.1
Define a list of valid keys, now that Rummager doesn't
Add a SearchIndex class

This is a thin wrap around a Rummageable::Index instance.
Make RummageableArtefact work with Rummageable 1.0

This replaces the existing calls to Rummageable in RummageableArtefact
to now request an instance of SearchIndex, and then call the new-style
Rummageable methods on that.
Fix test helpers