differentiate union and union-all #9
Description
currently union executes a union-all - every outer hull is checked against every other, even if two came from the subject, for example.
This causes really poor union performance (in the tens of ops/sec on the Huge.json test case, vs the ~480 ops/sec that intersect and subtract get on the same case).
A union-all method is currently required, as it used internally in union and intersect for dealing with holes, so I'm proposing that union-all should be a separate implementation from union. It need not be made public.