Compilation cache/incremental linking

[vegard]

I haven't quite decided which (if not both) is better: caching the result of compiling expressions in a big program so that we only have to recompile dependent expressions or allowing some sort of incremental linking of separate files. Possibly they can even be viewed as two sides of the same coin (as in both cases we would probably have to serialise and deserialise alreayd compiled code). I tend towards a preference for caching, since it seems more fine grained and would also handle the case where undoing/redoing small changes would otherwise have caused full recompilations.