Proposal: Tidy up and export PSQ from base

[Edward Z. Yang]

The new IO event manager implements a priority search queue internally.
This is a kind of handy data structure; in particular, some algorithmic
improvements in Hoopl would require a decent priority queue implementation.
I propose that we move GHC.Event.PSQ to Data.PSQ, cleaning up the dependency
on GHC.Event.Unique, and export it unconditionally (right now it is only
available on non-Windows.)

Discussion period: 1 week.

Cheers,
Edward