Proposal: Tidy up and export PSQ from base

[Edward Z. Yang]

Excerpts from Isaac Dupree's message of Sat Apr 30 12:24:47 -0400 2011:
> (After code cleanup), you *could* re-export base:GHC.Event.PSQ as 
> containers:Data.PSQ ... then if base ever stops needing it, you can copy 
> the implementation into containers (where I guess it logically belongs) 
> without breaking everything?

Yes, that would be a sensible option, which I quite like.

So I amend this proposal to also re-export this structure from containers.

Cheers,
Edward