Proposal: Tidy up and export PSQ from base

[Edward Z. Yang]

I think we should extend the discussion period for another week.

Cheers,
Edward

Excerpts from Edward Z. Yang's message of Fri Apr 29 21:10:41 -0400 2011:
> 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