Recursively compares a and b, returning a tuple of [things-only-in-a things-only-in-b things-in-both]. Comparison rules: * For equal a and b, return [nil nil a]. * Maps are subdiffed where keys match and values differ. * Sets are never subdiffed. * All sequential things are treated as associative collections by their indexes, with results returned as vectors. * Everything else (including strings!) is treated as an atom and compared for equality.