Added a copy of graph_functions called Graph_functions3 to implement
reachability with Shiabshis' new algorithm.
Removed some parts of the old algorithm, still implementing the new.
Wrote a lot of code for Graph_functions3 which is supposed to replace
Graph_functions and implement reachability.
Bunch of random changes supposed to implement a purely raw_t based
version of our algorithms.
Added a dependency on Graph_functions3.ml. There's going to be a lot
of cleaning up to do later.
Interface change in ZVGLT - added a function to combine two trees into
one tree.
Interface change in ZVGLT - added a function to combine two trees into
one tree.
Reverting "Interface change in ZVGLT - added a function to combine two trees into one tree."
Revert "Interface change in ZVGLT - added a function to combine two trees into"

This reverts commit 10fcf12793efb86e4111f57918916564aec3172d - which
was erroneously named.
Modified stuff to use Graph_functions3.ml
New functions, new tests.