Julia: move PriorityQueue out of Base?

[StefanKarpinski]

Before resolving issues like https://github.com/JuliaLang/julia/issues/4372, we should figure out if we really want something like a priority queue in Base Julia.

[ViralBShah]

Seems like something that could live in DataStructures.jl.

[timholy]

I sometimes wonder if it would be useful in some kind of work scheduler, once multithreading becomes more common. But that's pure speculation...