added a wireformat for HLists
added an indirection on Return nodes to avoid recursion into the node value when rewriting. fixes #266
better creation of Return.unit
added a weak hashset to store values when doing a distinct
added a weak hashset to store values when doing a distinct
added the necessary changes to compile with CDH3
Revert "added the necessary changes to compile with CDH3"

This reverts commit d89c5afb24cf7d1e53da335d2a51fb25ca876e04.
changed the cdh3 commit sha