postgres/v0.3.0

Changed

• Breaking: the provider supplies no dialect. The dialect type, its MapError
  classification into the base module's constraint classes, and its ast.ReturningRenderer
  capability are removed; the sqlate/postgres package owns the PostgreSQL dialect, and a
  composition root wraps the pool it constructs with sqlate.Wrap. New keeps its signature
  and calls the base module's New(conn, cfg).
• Breaking: the Provider constant is removed with the base module's Provider type.
• Requires github.com/standards-lab/go-database v0.4.0.