the cube of lisp booleans (#f nil () #t)

the cube of lisp booleans (#f nil () #t)

 * Renumbers the IFLAG constants.

 * Adds several macros related to boolean type tests, null tests, and
   boolean-truth testing (including lisp-style boolean-truth tests).

 * Adds compile-time checks to verify the necessary IFLAG numbering
   properties needed for the checks to work properly.

 * Changes some existing code to use the new optimized macros, without
   changing the semantics of the code at all (except that scm_is_bool
   is changed from a function to a macro).

I added the following macros, whose names explicitly state how %nil
should be handled.  See the comments in the patch for more information
about these.

  scm_is_false_assume_not_lisp_nil  scm_is_true_assume_not_lisp_nil
  scm_is_false_and_not_lisp_nil     scm_is_true_or_lisp_nil
  scm_is_false_or_lisp_nil          scm_is_true_and_not_lisp_nil

  scm_is_lisp_false                 scm_is_lisp_true

  scm_is_null_assume_not_lisp_nil
  scm_is_null_and_not_lisp_nil
  scm_is_null_or_lisp_nil

  scm_is_bool_and_not_lisp_nil
  scm_is_bool_or_lisp_nil

The following already-existing macros are defined as aliases, such
that their semantics is unchanged (although scm_is_bool used to be a
function and is now a macro).

  scm_is_null   -->  scm_is_null_and_not_lisp_nil
  scm_is_false  -->  scm_is_false_and_not_lisp_nil
  scm_is_true   -->  scm_is_true_or_lisp_nil
  scm_is_bool   -->  scm_is_bool_and_not_lisp_nil

(I still believe that these should be changed to versions that handle
 %nil properly, but await approval on that point, so these patches do
 not make those changes)

Also, if the preprocessor macro SCM_ENABLE_ELISP is not true (this
macro already existed and was used in lang.h), all overheads
associated with %nil handling are eliminated from the above macros.

* libguile/tags.h (SCM_BOOL_F, SCM_BOOL_T, SCM_UNSPECIFIED)
  (SCM_UNDEFINED, SCM_UNBOUND, SCM_ELISP_NIL): Renumber, so that a
  number of important distinctions (false versus true, end-of-list, etc)
  can be made by masking a single bit. Also define a number of
  build-time tests to assert that this condition holds.

* libguile/boolean.h (scm_is_false_and_not_nil, scm_is_true_or_nil)
  (scm_is_false_assume_not_nil, scm_is_true_assume_not_nil):
  (scm_is_false_or_nil, scm_is_true_and_not_nil)
  (scm_is_bool_or_nil, scm_is_bool_and_not_nil): New exciting macros to
  test certain boolean/end-of-list properties.
  (scm_is_false, scm_is_true): Use a restrictive definition, where only
  SCM_BOOL_F is false. Should probably change in the future.
  (scm_is_bool): Incompatible change: changed to be a macro. Was a
  function before. Probably should allow nil as a boolean, but that will
  be for a later patch.
  (scm_is_lisp_false, scm_is_lisp_true): New macros, implementing the
  standard Lisp boolean predicates, where '() is actually false.

* libguile/eval.i.c (CEVAL): Fix a number of false-or-nil and similar
  tests to use the new macros.

* libguile/lang.h (SCM_NULL_OR_NIL_P): Use scm_is_null_or_nil.

* libguile/pairs.c: Add a compile-time check that null and nil differ by
  only one bit.

* libguile/pairs.h (scm_is_null_and_not_nil, scm_is_null_assume_not_nil)
  (scm_is_null_or_nil): New exciting macros!
  (scm_is_null): Just be scm_is_null_and_not_nil, for now.

* libguile/print.c: Adapt to the reordering, and print suitably nasty
  things for the not-to-be-used values.