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.
If you wished to factor out the unpacks here, you could compose an "over each T.pack" somewhere in there. If you like lenses.