- Add 19 new messages. Update test outputs accordingly.

- Pretty print suggest-extensions hints: remove space before
interspersed commas.

- Refactor Rank's MonoType constructors. Each MonoType constructor
should represent a specific case. With the Doc suggestion belonging
to the TcRnMessage diagnostics instead.

- Move Rank from Validity to its own `GHC.Tc.Types.Rank` module.

- Remove the outdated `check_irred_pred` check.

- Remove the outdated duplication check in `check_valid_theta`, which
was subsumed by `redundant-constraints`.

- Add missing test cases for quantified-constraints/T16474 & th/T12387a.