is_fixnum(obj)
Return true if obj is a fixnum,
otherwise return false.
Fixnums represent exact integers within a closed range [-2w-1, 2w-2 - 1], where w is the fixnum width.
The implementation-specific value of w can be determined via the function
fixnum_width, and the endpoints of
the range may be determined via the functions least_fixnum and
greatest_fixnum.
is_fixnum(greatest_fixnum())
// true
is_fixnum(greatest_fixnum() + 1)
// false