Refactor roundabout way of returning Enumerator
Yield export data in batches
Expect `fetch_records` to yield values rather than return an array
Initial stab at persistent incrementers
Provide explicit `yields` method in bot to tie the bot and record together.

Means (for now) we can only yield one kind of record, but YAGNI and
cleans up the code.
Support fetching and exporting specific records
Rename to AsciiIncrementer
Rename variable
Depluralize `unique_fields`
Remove debug cruft
Add a ManualIncrementer
Include backports as dependency
Merge branch 'master' into enumerators-and-iterators
Refactor incrementers
Don't save in large batches when in test mode
Support multi-column indexes (requires upgrade to sqlite_magic)
Ensure interally-used columns exist in the export rather than run method
Allow `to_pipeline` to return nil
Support outputting "identifier" in company schema
Use version of sqlite_magic from git