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.

Makes the following methods of `std::net::Ipv4Addr` unstable const under the `const_ipv4` feature:
 - `is_global`
 - `is_reserved`
 - `is_broadcast`
 - `to_ipv6_compatible`
 - `to_ipv6_mapped`

This results in all methods of `Ipv4Addr` being const.

Also adds tests for these methods in a const context.