Constify the following methods of `std::net::Ipv4Addr`:
- `octets`
- `is_loopback`
- `is_private`
- `is_link_local`
- `is_shared`
- `is_ietf_protocol_assignment`
- `is_benchmarking`
- `is_multicast`
- `is_documentation`
Also insta-stabilizes these methods as const.
Possible because of the stabilization of const integer arithmetic and control flow.