tuned union: avoid garbage in trivial case;

dest_ctyp: raise exception for non-constructor; dest_comb: replaced expensive fastype_of by Term.argument_type; dest_abs: replaced expensive variant_abs by Term.dest_abs; hyps: fast_term_ord;

added fast_indexname_ord, fast_term_ord; changed sort_ord, typ_ord, Vartab, Termtab: use fast orders; added argument_type_of, dest_abs; tuned;

use fast_string_ord;