Implement changes specified in P0935R0.
There were a few constructors in the queue header which did not look right, so I updated them using my best judgment.
Edit: also, I used _LIBCPP_INLINE_VISIBILITY where it looked like other's were using it. What is the rule of thumb for _LIBCPP_INLINE_VISIBILITY?
Edit 2: I didn't write any tests for this patch because I thought it wasn't needed (and because that would take a lot of time :P), but if you think it would be a good idea I can test for constructors blocking copy-list-initialization (or other things if necessary).