http://api.jquery.com/first-selector/ mentions that using ":first" in
query selectors can be a little slow since we can't use the browser's
native querySelectorForAll implementation.
In my tests, this cut the average time down from 6.70ms to 6.38ms.
So, not great, but this function is most frequently called
many times in a big while loop, so, better than nothing.
(imported from commit d7725edd0c81431829fe353c6dd2bc61b1de6b19)
I would like to have split this into several commits, but it's hard to do
incrementally, because functions need to be defined before their use in global
data.
(imported from commit 31533396b236f3b657950b073982c317256f082a)
