allow iflags to be constant expressions with typing-strictness==2

allow iflags to be constant expressions with typing-strictness==2

* libguile/tags.h (SCM_MAKE_ITAG8_BITS): New helper, produces a
  scm_t_bits instead of a SCM, because SCM_UNPACK is not a constant
  expression with SCM_DEBUG_TYPING_STRICTNESS==2.
  (SCM_MAKIFLAG_BITS): Remove SCM_MAKIFLAG, and replace with this, which
  returns bits.
  (SCM_BOOL_F_BITS, SCM_ELISP_NIL_BITS, SCM_EOL_BITS, SCM_BOOL_T_BITS):
  (SCM_UNSPECIFIED_BITS, SCM_UNDEFINED_BITS, SCM_EOF_VAL_BITS):
  (SCM_UNBOUND_BITS): New definitions.  Defined SCM_BOOL_F, etc in terms
  of them.
  (SCM_XXX_ANOTHER_BOOLEAN_DONT_USE_0):
  (SCM_XXX_ANOTHER_BOOLEAN_DONT_USE_1):
  (SCM_XXX_ANOTHER_BOOLEAN_DONT_USE_2):
  (SCM_XXX_ANOTHER_LISP_FALSE_DONT_USE): Be bits instead of SCM values.
  (SCM_BITS_DIFFER_IN_EXACTLY_ONE_BIT_POSITION):
  (SCM_BITS_DIFFER_IN_EXACTLY_TWO_BIT_POSITIONS): Rename from
  SCM_VALUES_DIFFER_..., and take unpacked bits as the args.

* libguile/boolean.c: Update verify block to use
  SCM_BITS_DIFFER_IN_EXACTLY_TWO_BIT_POSITIONS et al.

* libguile/debug.c (scm_debug_opts):
* libguile/print.c (scm_print_opts):
* libguile/read.c (scm_read_opts): Use iflags bits for initializers.

* libguile/hash.c (scm_hasher): Use _BITS for iflags as case labels.

* libguile/pairs.c: Nil/null compile-time check uses
  SCM_ELISP_NIL_BITS.