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:

- rust-lang#27709
- rust-lang#76205

…oerce to the fn return type

…ebank

Suggest adding `return` if the for semi which can coerce to the fn return type

Fixes rust-lang#86094
r? `@estebank`

Stabilize `{IpAddr, Ipv6Addr}::to_canonical`

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

Newly stable API:

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

impl Ipv6Addr {
    // Newly 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:

- rust-lang#27709
- rust-lang#76205

Stabilization of all methods under the `ip` gate was attempted once at rust-lang#66584 then again at rust-lang#76098. These were not successful because there are still unknowns about `is_documentation` `is_benchmarking` and similar; `to_canonical` is much more straightforward.

I have looked and could not find any known issues with `to_canonical`. These were added in 2021 in rust-lang#87708

cc implementor ``@the8472``

r? libs-api
``@rustbot`` label +T-libs-api +needs-fcp

…ested, r=Mark-Simulacrum

Enable `review-requested` feature for rustbot

See rust-lang/triagebot#1733.

r? `@Mark-Simulacrum`