* Add DocPredicateQuery and a generic FunctionPredicate implementation
Introduces the DocPredicate/SegmentDocPredicate abstraction and
DocPredicateQuery, a query that walks a segment's documents by
repeatedly evaluating a per-segment predicate (with a scorer_danger
fast path for boolean intersections). FunctionPredicate is a first,
generic implementation built from a plain per-segment factory
function, with no dependency on fast fields or any other segment
data structure.
* CR comments
* Better explain
