SERVER-8581 use $$DESCEND rather than $$CONTINUE in $redact
 SERVER-10628 change $any to $anyElementTrue, $all to $allElementsTrue, and remove $none
SERVER-10086 fix $setDifference to return a set (spec changed)
SERVER-10588 $cond accepts an object with the fields if, then, and else (a la $map and $let)
SERVER-8581 add $$KEEP example to $redact