Remove remnants of `recursively` from public API

[Jolanrensen]

all() still returns a TransformableColumnSet<C> in the selection DSL as well as several others. The only reason this public type was different was the plan to allow post-fix modifications on column selections, like all().recursively(). Since we found this to be counterintuitive, we should change the public types back to ColumnSet (and perhaps clean up internal usage as well).

[zaleslaw]

@Jolanrensen Beta1 or 2?

[Jolanrensen]

2 or later. It's not publicly visible, just a bit ugly and cumbersome