WIP: cleanup (again)

[lue-bird]

• [x] Introduce more general checks for sequence, join, and repeatFlat
• [x] Introduce MappableProperties
• [x] Use "can be combined checks" more
• [x] Organize helpers better into sections
• [x] Introduce helpers to create type subset properties

to discuss

• I'd like to rename "andThen" checks to "mapFlat" which is in my mind easier to understand as concatMap and has a nice conststency with e.g. "repeatFlat" which is effectively repeat >> andThen identity.

[jfmengels]

You have some conflicts because of my restoration of old PR (sorry :see_no_evil:). Let me know if you want some help with that.

I'd like to rename "andThen" checks to "mapFlat"

Go ahead. These names are all quite abstract at this point anyway (but not Haskell-like abstract).