Generic flattening API?

## What is your idea?

Ruler comes with a built-in flattener for JSON. Suppose I wanted one for Avro or Protobufs or whatever. It might be nice to have a generic Flattening interface that other people could implement for whatever data type. It should include the ArrayMembership apparatus because other data types are apt to have arrays too.

## Would you be willing to make the change?

Yes.

Start by reading the built-in JSON flattener and the ArrayMembership apparatus mentioned in the issue. Compare their responsibilities and determine what a generic Flattening interface would need to support JSON, Avro, Protobufs, and array membership. Done means the interface design is agreed and other data types can implement it.

Written by the indexing model from the issue text.