Exports of STM constructors.

[Simon Marlow]

Simon Peyton-Jones wrote:

> Your suggestion seems plausible to me:
> 
> -          export *everything* from GHC.Conc; import this only if you 
> know what you are doing
> 
> -          export STM, TVar only *abstractly* from 
> Control.Concurrent.STM; this is the interface for ordinary users
> 
>  
> 
> This sounds like a candidate for the standard library modification 
> process, if someone wants to put it into action.
> 
>                 http://haskell.org/haskellwiki/Library_submissions

I think GHC.Conc is private to GHC, not an official API, and therefore not 
subject to the library submission process.  I'll go ahead make this change.

Cheers,
	Simon