extend_theory: changed type of "abbrs" arg;
added cterm_fun, read_ctyp (from drule.ML);
ctyp_of, cterm_of, etc.: now use Sign.certify_...;
assumption: now uses Envir.is_empty;
bicompose_aux: fixed BUG (unifier with empty "asol" but non-empty "iTs"
wasn't applied);
fixed axioms_of;