Alpha-2 frozen

[<eric@son-of-blob>]

I just froze my alpha-2 directory and put a symbolic link to it as
/usr/local/share/xanadu/backend/alpha-2.

Alpha-2 has iterators that seem to work.  The include files for them
are in alpha-2/iterators.  The iterator object is linked into the *Xpp*.o
libraries.  Alpha-2 also contains an Iterator implementation of Tuples,
called IteratorTuple, and two hash table based set classes

 CLASS(HashSetBang, Object) and CLASS(HashSet, Set)

that use a growable hash table.  A future alpha will actually use these
classes.  Alpha-2 also contains a 2.0 compatible descendent of Walker's
smart allocation stuff (switched off in the release) and 2.0 tombstones.
Note that tombstones aren't as nice in 2.0 as the destructors throw away
the original vtable pointer as the base class destructors execute.