I have found that tree-flow.h implements iteration over htab_t,
while there is no current facility to do that with hash_table.
Unfortunately, the specific form does not match the standard C++
approach to iterators. We have several choices.
(1) Ignore the problem and leave all such tables as htab_t.
(2) Write new hash_table iteration functions to match the form of
the existing GCC macro/function approach.
(3) Write new hash_table iteration functions to match the form used
by the C++ standard. This approach would entail modifying the loops.
Diego and I have a preference for (3). What do you prefer?