defined take/drop by induction over list rather than nat.

Improving space efficiency of inductive/datatype definitions. Reduce usage of "open" and change struct open X; D end to let open X in struct D end end whenever possible -- removes X from the final structure. Especially needed for functors Intr_elim and Indrule. intr_elim.ML and constructor.ML now use a common Su.free_SEs instead of generating a new one. Inductive defs no longer export sumprod_free_SEs ZF/intr_elim: Removed unfold:thm from signature INTR_ELIM. It is never used outside, is easily recovered using bnd_mono and def_lfp_Tarski, and takes up considerable store. Moved raw_induct and rec_names to separate signature INTR_ELIM_AUX, for items no longer exported. mutual_induct is simply "True" unless it is going to be significantly different from induct -- either because there is mutual recursion or because it involves tuples.

Addition of compression, that is, sharing. Uses refs and Symtab (binary search trees) to get reasonable speed. Types and terms can be compressed.