cleanup for Fun.thy:
merged Update.{thy|ML} into Fun.{thy|ML}
moved o_def from HOL.thy to Fun.thy
added Id_def to Fun.thy
moved image_compose from Set.ML to Fun.ML
moved o_apply and o_assoc from simpdata.ML to Fun.ML
moved fun_upd_same and fun_upd_other (from Map.ML) to Fun.ML
added fun_upd_twist to Fun.ML
Inductive = Gfp + Prod + Sum