This may seem like an odd change, but it simplifies the logic a lot. I kept having problems with passes not operating on externals (e.g. functions-to-procs, adding array sizes, constant folding in array dimensions) and adding a special case every time to also process the externals was getting silly.
Putting the externals in the AST therefore made sense, but I didn't want to just add dummy bodies as this would cause them to throw up errors (e.g. in the type-checking for functions). So I turned the bodies into a Maybe type, and that has worked out well.
I also stopped storing the formals in csExternals (since they are now in csNames, and the tree), which streamlined that nicely, and stopped me having to keep them up to date.
One of these bugs is that array variables are counted as unused when they are used subscripted. I think that should be solved when we flip back to the listify approach.
The second bug is more interesting, as it is triggered only in a certain arrangement with an IF. It's either a bug in the flow-graph building or in the varsTouchedAfter code.
