use guile conses

use guile conses

* src/alloc.c (Fcons): Use `scm_cons'.

* src/lisp.h (lisp_cons_tag, XCONS, XSETCONS, struct Lisp_Cons)
  (XCAR_AS_LVALUE, XCDR_AS_LVALUE): Remove.

  (XCAR): Use `scm_car'.
  (XCDR): Use` scm_cdr'.
  (XSETCAR): Use `scm_set_car_x'.
  (XSETCDR): Use `scm_set_cdr_x'.
  (CONSP): Use `scm_is_pair'.

* src/undo.c (truncate_undo_list): Use `sizeof (scm_t_cell)' as the size
  of a cons cell for estimating memory usage.