Stop exporting the not implemented CompressedFile in Open.oz

* The ZlibIO boot module is not yet implemented.
* A few scripts (Pickler.oz, Executor.oz) check
  {HasFeature Open compressedFile},
  so now they should behave properly as it is not defined.
Merge pull request #189 from eregon/zlibio

Stop exporting the not implemented CompressedFile in Open.oz