ZF/ZF.ML/UN_iff, INT_iff: added to the signature

Pure/thm/norm_term_skip: new, for skipping normalization of the context Pure/thm/bicompose_aux: now computes nlift (number of lifted assumptions in new subgoals) and avoids normalizing the first nlift assumptions in the case where the proof state is not affected. Pure/thm/norm_term_skip: now normalizes types of parameters Pure/thm/THM: aligned colons

Pure/sequence: added comment explaining that memoing sequences were found to be much slower (due to overheads) than the sequences currently used.