Add deep subsumption
Addressing #21548 (closed), this MR adds DeepSubsumption, which un-does (some of) the "Simplified Subsumption" change. It has a similar flavour as NoMonoLocalBinds: it makes type inference more complicated and less predictable, but it may be convenient in practice.
I regard this as experimental for now. But it has the merit of simplicity: the changes are highly localised.
I did not change Quick Look at all.