Proposal: Tidy up and export PSQ from base

[Edward Z. Yang]

Excerpts from Antoine Latter's message of Fri Apr 29 21:25:13 -0400 2011:
> What will you also export GHC.Event.Unique?

My inclination is no, although I'm willing to be convinced otherwise.
The PSQ in this case would be more like an IntMap (call it IntPSQ?)

One consideration is the fact that GHC.Event.Unique currently unconditionally
uses Int64, where it seems to me that you'd want to use whatever the native
Int is per platform--someone who worked on the new event manager care to
elucidate?

Edward