Ref

@jserranohidalgo I've been cleaning the core (*Overreact.hs*) and the programs ([*GCD*, *Quicksort*, *ZipWithIndex*].hs). I've moved the `data Ref` to another file (*Ref.hs*) as a temporal solution, in order to put all the junk together in the interpreter module. I think it's good enough for our current purposes but I'll wait to merge until you send me the OK, because perhaps you want to keep it even cleaner.