One-shot semantics in GHC event manager

[Kazu Yamamoto (=?iso-2022-jp?B?GyRCOzNLXE9CSScbKEI=?=)]

> I already pushed it. The commit in question is
> 5dce47eb8415eb31e1c6759b6f6a2ef5bfe32470. Thanks for the benchmarking!

I believe this is in bgamari/ghc (for GHC 7.10?).
I'm using bgamari/packages-base for GHC 7.8 and asking to push the
same commit to this repo.

Actually I compared the latest Internal.hs in bgamari/ghc and
one in bgamari/packages-base and I saw *additional* differences.
So, I hesitated to apply the patch by myself.

--Kazu