Fix comparaison with large IPSet()

`__lt__` and `__gt__` use `len()`
```
>>> a = IPSet(['fc00::/7'])
>>> b = IPSet(['fe80::/10'])
>>> a < b
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/yohann/netaddr_play/src/netaddr/netaddr/ip/sets.py", line 425, in __lt__
    return len(self) < len(other) and self.issubset(other)
  File "/home/yohann/netaddr_play/src/netaddr/netaddr/ip/sets.py", line 614, in __len__
    "IP addresses! Use the .size property instead." % _sys_maxint)
IndexError: range contains greater than 2147483647 (maxint) IP addresses! Use the .size property instead.
```
This fix replaces `len()` with  the .`size` attribute.