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.