Known issues:
- `PhutilIPAddress` only supports IPv4.
- `PhutilCIDRBlock` only supports IPv4.
- Double-check for remaining callers to `ip2long()` + integer IP address storage, and/or short address fields in the database. I believe we removed all of these already in anticipation of IPv6 support.
- Possibly need to deal with this junk in URIs? `http://[2001:db8:85a3:8d3:1319:8a2e:370:7348]/`