Remove remnants of `recursively` from public API

`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).