Remove circular includes

[johnmcfarlane]

What would you like to see in CNL?

Source files likely have many circular #includes which are not apparent because of header guards. Nevertheless, they are not nice. It's a crapshoot what order a lot of stuff gets defined in. A cleaner #include graph would be easier to reason about and to manage.

See if this comment speaks the truth. See if CNL can be made to work this way.