Fix for CAF retention when dynamically loading & unloading code
In a situaion where we have some statically-linked code and we want to load and unload a series of objects, we need the CAFs in the statically-linked code to be retained indefinitely, while the CAFs in the dynamically-linked code should be GC'd as normal, so that we can detect when the code is unloadable. This was wrong before - we GC'd CAFs in the static code, leading to a crash in the rare case where we use a CAF, GC it, and then load a new object that uses it again. I also did some tidy up: RtsConfig now has a field keep_cafs to indicate whether we want CAFs to be retained in static code.
Showing with 50 additions and 13 deletions