Replace iterators with C++-11 for loops

These are significantly cleaner and probably a bit faster as well!
Note that I'm making this change backwards-incompatible because I'm
going to go ahead and make a whole series of backwards-incompatible
changes.
Add flo-memcheck

This processes the input Flo file a whole bunch of times.  The general
idea is to ensure that libflo doesn't leak memory.