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.
This means all the tests now get run as part of one list, and HUnit keeps track
of the number of failures for us. (The reason I was doing this was so that
tocktest will exit non-zero on QuickCheck test failure too.)
As part of this, I've reworked TestMain's main function quite a bit. It'll now
filter QuickCheck tests into response to options too.
