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
Load Pickle in the VM module only in VM.getPort and VM.new.

* So it only loaded when we are about to need it
  (we cannot easily load it in Send).
* If someone uses a reference to VM.getPort, then passing it around
  caries the dependency on Pickle, which is necessary.
* Messages sent to monitor streams are fine as they do not need Pickle.