Made rts_evalIO() stricter, i.e.,

   rts_evalIO( action );

will now essentially cause `action' to be applied
to the following (imaginary) defn of `evalIO':

    evalIO :: IO a -> IO a
    evalIO action = action >>= \ x -> x `seq` return x

instead of just

    evalIO :: IO a -> IO a
    evalIO action = action >>= \ x -> return x

The old, lazier behaviour is now available via rts_evalLazyIO().