patch applied (packages/base): Avoid strictness in accumulator for unpackFoldr

[Don Stewart]

Mon Jul  3 02:18:06 PDT 2006  Don Stewart <dons at cse.unsw.edu.au>
  * Avoid strictness in accumulator for unpackFoldr
  
  The seq on the accumulator for unpackFoldr will break in the presence of
  head/build rewrite rules. The empty list case will be forced, producing
  an exception. This is a known issue with seq and rewrite rules that we
  just stumbled on to.
  

    M ./Data/ByteString.hs -16 +21