Stabilize `{IpAddr, Ipv6Addr}::to_canonical`

Make `IpAddr::to_canonical` and `IpV6Addr::to_canonical` stable, as well as
const stabilize `Ipv6Addr::to_ipv4_mapped`.

Newly stable API:

    impl IpAddr {
        // Now stable under `ip_to_canonical`
        const fn to_canonical(&self) -> IpAddr;
    }

    impl Ipv6Addr {
        // Now stable under `ip_to_canonical`
        const fn to_canonical(&self) -> IpAddr;

        // Already stable, this makes it const stable under
        // `const_ipv6_to_ipv4_mapped`
        const fn to_ipv4_mapped(&self) -> Option<Ipv4Addr>
    }

These stabilize a subset of the following tracking issues:

- https://github.com/rust-lang/rust/issues/27709
- https://github.com/rust-lang/rust/issues/76205

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