This is a draft of spatial 'intersects'.

The simplified workflow outlines as such (vars in upper case and relations
start with a capital this time):
- In case of Contains/Intersects, split posted geometry into dim-2
  (->RELATED_GEOJSON) and dim-1,0 objects (->SKIPPED).
- In case of Within, store the entire posted geometry into RELATED_GEOJSON.
- In case of Contains/Intersects, determine one point and its side-ness w/r/t
  RELATED_GEOJSON.
- In case of Contains, set RELATES to a function that returns true, only if all
  points of a given GEOJSON are inside RELATED_GEOJSON. As soon as one point is
  found to be outside RELATED_GEOJSON, RELATES quits early and returns false.
- In case of Intersects, set OUTSIDE_RELATED_GEOJSON to a function that returns
  true, if all points of GEOJSON lie outside RELATED_GEOJSON; This is the same
  function as above, just with the side-ness flipped (neat).
- In case of Within, set RELATES to a function that returns true if all points
  of RELATED_GEOJSON lie inside a given GEOJSON.
- In case of Intersects, set SKIPPED_OUTSIDE to a function that returns true if
  all points of SKIPPED lie outside a given GEOJSON; Again, this is the same as
  above, save flipped sideness and different set of points.
- In case of Intersects, set RELATES TO
    GEOJSON --> !OUTSIDE_RELATED_GEOJSON(GEOJSON) || !SKIPPED_OUTSIDE(GEOJSON)
  because !OUTSIDE_RELATED_GEOJSON states that at least one point of GeoJSON is
  inside RELATED_GEOJSON and this already qualifies as intersection (ditto for
  the other part).
Second draft, slightly different workflow.

- Instead of storing LineStrings etc. in SKIPPED, I stored the entire posted
  geometry as RELATED_GEOJSON regardless of the requested relation and it
  remains unaltered. Then I copy all the dim-2 objects out of it into
  RELATED_POLYGONS.
  This version is probably more secure because the previous
  version assumed that, having no points of polygon B within polygon A means
  that no points of polygon A are in polygon B as well.
- Stripped and un-stripped polygons are properly taken care of. The former
  completely spoil point-in-polygon checks, whereas the latter may cause
  end-points to be checked twice.
Removed some redundant anonymous function brackets.