(scm_is_signed_integer, scm_is_unsigned_integer):

(scm_is_signed_integer, scm_is_unsigned_integer):
Rewritten using the same logic as scm_to_signed_integer and
scm_to_unsigned_integer, respectively, which is better(tm).  Also,
use CHAR_BIT instead of hardcoding 8.
(LLONG_MIN, LLONG_MAX, ULLONG_MAX): Removed and used
SCM_I_LLONG_MIN etc. instead.

* numbers.h (SCM_MAKINUM, SCM_I_MAKINUM): Renamed SCM_MAKINUM to
SCM_I_MAKINUM and changed all uses.