- Move IPRange into its own net/ip_range.hpp
- Move the static net::IPPacket::TruncateV6, etc. functions to free
net::TruncateV6, etc. functions (now from net/ip.hpp instead of
net/ip_packet.hpp).
- Make net::TruncateV6 and net::ExpandV4 constexpr.
- Add IPRange::FromIPv4 factory function (to replace the iprange_ipv4
free function)
Rename net/ip.{cpp,hpp} to net/ip_packet.{cpp,hpp}.
(Doing this in two commits because I want to repurpose ip.hpp/ip.cpp,
and want git to figure out the history properly).
