Call SUPER::DESTROY in DESTROY.
t/10-construction.t: add various cases to construct Future::Q objects. The test is failing because Future's bug.
See https://rt.cpan.org/Public/Bug/Display.html?id=97537
Q: SUPER::wait_all() etc. are now called via code-refs kept in advance, instead of calling can() method every time. Performance boost of 5 - 15% confirmed.
Q: is_rejected: boolify the result of failure() method. This is safer.