Proposal: Add tryReadChan to Chan

[Mitar]

Hi!

Now that isEmptyChan is deprecated (#4154) it would be useful to add
tryReadChan. It seems it is possible to define it so that it does not
block like isEmptyChan. This is because semantics is a bit different:
tryReadChan is allowed to return Nothing also when Chan is non-empty,
but it would block. Contrary, isEmptyChan is expected that it would
return False only if Chan is really empty.

I have defined also two new MVar functions which helped me
implementing tryReadChan: tryReadMVar and tryModifyMVar. Probably it
would be useful to make them public.

Please comment. I propose three weeks for discussion.

I have also opened a ticket:

http://hackage.haskell.org/trac/ghc/ticket/4535

and attached a ptach there with working implementation. It would be
useful to run it against some tests.

(I am sorry that patch is not in darcs format.)


Mitar