Don't try to run finalizers at program exit.  This turned out to be
hard if not impossible to do in general, so now we don't attempt it at
all.

The Main.main wrapper, previously called runIO and now called
runMainIO, flushes stdout and stderr before exiting.  This should
catch most cases where programs rely on Handles being flushed at
program exit, but note that now if you simply drop a Handle in your
program, there's no guarantee it'll be flushed on exit.  If the
punters complain enough, I suppose we could implement a global
Handle table and flush them all at exit... I'd rather not do this if
possible, though.  Better to teach people to close their Handles
properly.