One-shot semantics in GHC event manager

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

> Well, Bas was wondering whether this would be possible. At this point
> I'm a bit on the fence; on one hand it's not a crucial fix (we have a
> workaround in usb) and it may involve changes to exported interfaces
> (although not very high visibility). On the other hand, it's a pretty
> easy change to make and it cleans up the semantics of the event manager
> nicely.

I benchmarked your patches on GHC 7.8 branch:

-N<n>          1       2        4        8        16
---------------------------------------------------------
head           92,855  155,957  306,813  498,613  527,034
7.8.3          86,494  160,321  310,675  494,020  510,751
7.8.3+ben      84,472  140,978  291,550  488,834  523,837

The inline patch works very nice. :-)

# And I was disappointed a bit because GHC does not automatically do
# this inline.

--Kazu