Sketching criterion for pre-filter to _view/fields-values.

I've included _keyword as well as field/value because even _keyword might run
faster in some cases if I crafted this in an awesome way.
- _expressions are no use to pre-filter in any case.
- Single fields over multiple types are also no use to pre-filter; I suspect
  _view/fields-values to run over too many unused rows.
WIP: use _list/tabulate/fields-values to pre-filter large document sets.