Make all methods of `std::net::Ipv6Addr` const
Make the following methods of `std::net::Ipv6Addr` unstable const under the `const_ipv6` feature:
- `segments`
- `is_unspecified`
- `is_loopback`
- `is_global` (unstable)
- `is_unique_local`
- `is_unicast_link_local_strict`
- `is_documentation`
- `multicast_scope`
- `is_multicast`
- `to_ipv4_mapped`
- `to_ipv4`
This would make all methods of `Ipv6Addr` const.
Changed the implementation of `is_unspecified` and `is_loopback` to use a `match` instead of `==`, all other methods did not require a change.
All these methods are dependent on `segments`, the current implementation of which requires unstable `const_fn_transmute` ([PR#75085](https://github.com/rust-lang/rust/pull/75085)).
Part of #76205
11ff32f9ec