On reflection, using the '!!' prefix for unboxing a constructor field
was a mistake:  firstly it has to be written '! !', and secondly it
isn't backwards-compatible.  So instead, we now use the syntax:

   data T = T {-# UNPACK #-} !Int

to unbox a field.  We thought this would be a cute reuse of the UNPACK
pragma.

The effect of -funbox-strict-fields is to add {-# UNPACK #-} to every
strict field.