Fix performance regression: re-instate -funbox-strict-fields

Fix performance regression: re-instate -funbox-strict-fields
Yikes!  While investigating the increase in code size with GHC 6.8
relative to 6.6, I noticed that in the transition to Cabal for the
libraries we lost -funbox-strict-fields, which is more or less
depended on by the IO library for performance.  I'm astonished that we
didn't notice this earlier!

To reduce the chances of this happening again, I put
-funbox-strict-fields in the OPTIONS_GHC pragma of the modules that
need it.  {-# UNPACK #-} pragmas would be better, though.