Equipped start_key and end_key to _list/tabulate/type-year.

It doesn't do anything if all types are selected, but it greatly restricts the
amount type-year keys if nearby types are selected.
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:
- Document counts of less than 100 won't be pre-filtered.
- _expressions are no use to pre-filter in any case.
- Single fields over multiple types might not be good to pre-filter; I suspect
  _view/fields-values to run over too many unused rows. On the other hand if
  you have a bunch of very similar documents, who knows?
- If the mere existence of a field has to be known, group_level expands over
  [<type>, <field>] to improve _list response time.
Regardless of these precautions, it is still essential ensuring the number of
rows processed by _list/tabulate to not exceed the number of documents.
However, because I don't see a safe and easy way to count the number of unique
keys, I placed a limit of 1 and skipped over the number of rows I set as a
threshold.
Dropped variant group_level in pre-filter query params.

Simply because _list/tabulate doesn't yield the keys that have to be forwarded
to _view/fields-values. They have to match exactly and they don't when the last
element is missing.
Single-quoted cut.field if it isn't _keyword.

Unfortunately row.value.doc still has flat-hierarchy fields and I cannot
inflate them because in the reduce function I am unable to
require('view/lib/path').
This spoils my beautiful code symmetry and another day is ruined.
Truncated last coordinate digit in utils/simplify().
Removed empty cuts array from options.

The empty cuts array caused an exception when filter_documents was looking for
cuts[0].remove_me.
Rearranged new_filter.redo to the left.
WIP: remove stupid pre-filter quirks. this isn't ready by far.