Add support for Invoices

This branch adds support for invoices.

Status:

- `anchor-api-migrate` includes a table for invoices
- `anchor-apid` provides `/v1.0.0/invoice` endpoints which serve this data
- un/parsing from JSON works on test data.

Todo:

- synch the invoices into the database (to be added to `anchor-retcon`, not here).

So this is ready for review and merge.
I say "force" because I didn't check if this is legit, but it does seem to work for the half-dozen or so specific numbers in the JSON document it tried it with.