Split out rel8-schema library

[TeofilC]

The plan is that we will have:

• rel8-internal: exposes all modules
• rel8: exposes the current interface
• rel8-schema: defines schema handling logic like the stuff from Schema type checker #335

Does that sound right @ocharles ?

[ocharles]

I'm happy with that!

[isovector]

This sounds very desirable. I was just coming to ask if we can have access to Rel8.Schema.QualifiedName; my use case being that I'd like to use the rel8 schema as my source of truth and define PKs on top of it:

data DbConstraint table where
  PK :: (Rel8.TableSchema table -> Rel8.Name a) -> DbConstraint table

mkConstraints :: Rel8.TableSchema table -> DbConstraint table -> Session ()
mkConstraints (Rel8.TableSchema name x) (PK f) =
  sql $ BS8.pack $ unwords
    [ "ALTER TABLE"
    , showQualifiedName name  -- not exported; oh no!!!
    , ...
    ]

[TeofilC]

Nice! I'll try to get around to do this soon. (Or PRs would also be appreciated)