Replace cidr_merge algorithm with something that doesn't use regexes.
Replace cidr_exclude with a more efficient method based on cidr_partition.
A faster iprange_to_cidrs using the new cidr_partition.

This has direct impact on all the IPSet() methods which use this function
extensively.
A faster spanning_cidr, now it's used a lot.
Merge pull request #54 from kleptog/fasteripset

Some algorithmic speedups