785 colsubsetexpr refactor

[gmbecker]

Not sure what happened to the original PR. I guess it got destroyed when I rebased on main after doing the inline commit thing (which sadly messed everything up by making things out of sink)

Sorry about but I think the (now lost) comments are addressed here...

[gmbecker]

@jcheng5

[Melkiades]

@gmbecker from tests:

Undocumented arguments in documentation object 'make_split_result'
  ‘sub_expr’

[gmbecker]

This is subsumed into #865