Geometry predicates should be able to prune on their own

[Mmoncadaisla]

Today a geometry-only predicate (ST_Within(geom, ...)) scans and encodes every chunk: engines do not push ST_* function predicates into the pyarrow scan, so nothing reaches the chunk pruner. The documented idiom is pairing the predicate with xql.bbox_conjuncts(...).

It would be nice if geometry predicates pruned without the manual pairing. Rough directions per path:

Raised in #227 (comment)