Scope as backend with new pg client

[Kamirus]

Major changes:

• Scope: the type parameter s in e.g. Col s a or Query s { | r } now also determines the backend used (now: whether PG or SQLite3) so that user can write more backend-agnostic queries
• type class Lit includes backend
  • this means that lit can be used instead of litPG when the desired backend is PG
  • also lit can be used in a backend-agnostic query and when executed an appropriate litPG or litSQLite3 will be dispatched
• dodo-printer is now used as a pretty printer
• environment variable-like information got separated from the code and now resides in .env (or env variables)

Before merging:

• every spago dependency should be in the package set

@paluh do we need all three polyform* dependencies?

[paluh]

I think that when I publish all these libs we need only polyform-batteries-env because it is going to pull its own deps by itself. I hope to do this finally tomorrow.