Avoid OverflowError with large ranges

Use `IpRange.__len__()` directly rather than calling `len(IpRange())` when
computing the length of an IpRangeList. This won't stop OverflowErrors in
calling code such as `len(IpRangeList('fe80::/10'))`, but it will provide
a method to work around the overflow by direct use of `IpRangeList.__len__()`.

Resolves: #12