`beam-migrate simple schema` fails on all but empty database

Based on instructions at #27 I run `stack exec -- beam-migrate simple schema --backend Database.Beam.Postgres.Migrate --connection "dbname= user= password="` to create a schema from an existing database, but:

- Running on my full (complex) postgres database results in `beam-migrate` occupying all available memory then coredumping.
- Running on a test database with even one table results in `beam-migrate: user error (Could not form haskell schema)`
- Running on a completely empty test database seems to work OK.

Using beam master branch and Postgresql 10.4 on Arch linux.