Proposal: Tidy up and export PSQ from base

[Edward Z. Yang]

Excerpts from Johan Tibell's message of Sun May 01 15:19:55 -0400 2011:
> Yes. And the values as well (i.e. monomorphic in the keys, priorities,
> and values).

Wait, values too?  Doesn't look like it in the source...

-- | @E k p@ binds the key @k@ with the priority @p at .
data Elem a = E
    { key   :: {-# UNPACK #-} !Key
    , prio  :: {-# UNPACK #-} !Prio
    , value :: a
    } deriving (Eq, Show)

Cheers,
Edward