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:

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

library/core/src/net/ip_addr.rs

@@ -410,21 +410,24 @@ impl IpAddr {
     /// # Examples
     ///
     /// ```
-    /// #![feature(ip)]
     /// use std::net::{IpAddr, Ipv4Addr, Ipv6Addr};
     ///
+    /// let localhost_v4 = Ipv4Addr::new(127, 0, 0, 1);
+    ///
+    /// assert_eq!(IpAddr::V4(localhost_v4).to_canonical(), localhost_v4);
+    /// assert_eq!(IpAddr::V6(localhost_v4.to_ipv6_mapped()).to_canonical(), localhost_v4);
     /// assert_eq!(IpAddr::V4(Ipv4Addr::new(127, 0, 0, 1)).to_canonical().is_loopback(), true);
     /// assert_eq!(IpAddr::V6(Ipv6Addr::new(0, 0, 0, 0, 0, 0xffff, 0x7f00, 0x1)).is_loopback(), false);
     /// assert_eq!(IpAddr::V6(Ipv6Addr::new(0, 0, 0, 0, 0, 0xffff, 0x7f00, 0x1)).to_canonical().is_loopback(), true);
     /// ```
     #[inline]
     #[must_use = "this returns the result of the operation, \
                   without modifying the original"]
-    #[rustc_const_unstable(feature = "const_ip", issue = "76205")]
-    #[unstable(feature = "ip", issue = "27709")]
+    #[stable(feature = "ip_to_canonical", since = "CURRENT_RUSTC_VERSION")]
+    #[rustc_const_stable(feature = "ip_to_canonical", since = "CURRENT_RUSTC_VERSION")]
     pub const fn to_canonical(&self) -> IpAddr {
         match self {
-            &v4 @ IpAddr::V4(_) => v4,
+            IpAddr::V4(_) => *self,
             IpAddr::V6(v6) => v6.to_canonical(),
         }
     }
@@ -1748,11 +1751,11 @@ impl Ipv6Addr {
     ///            Some(Ipv4Addr::new(192, 10, 2, 255)));
     /// assert_eq!(Ipv6Addr::new(0, 0, 0, 0, 0, 0, 0, 1).to_ipv4_mapped(), None);
     /// ```
-    #[rustc_const_unstable(feature = "const_ipv6", issue = "76205")]
-    #[stable(feature = "ipv6_to_ipv4_mapped", since = "1.63.0")]
+    #[inline]
     #[must_use = "this returns the result of the operation, \
                   without modifying the original"]
-    #[inline]
+    #[stable(feature = "ipv6_to_ipv4_mapped", since = "1.63.0")]
+    #[rustc_const_stable(feature = "const_ipv6_to_ipv4_mapped", since = "CURRENT_RUSTC_VERSION")]
     pub const fn to_ipv4_mapped(&self) -> Option<Ipv4Addr> {
         match self.octets() {
             [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0xff, 0xff, a, b, c, d] => {
@@ -1817,11 +1820,11 @@ impl Ipv6Addr {
     /// assert_eq!(Ipv6Addr::new(0, 0, 0, 0, 0, 0xffff, 0x7f00, 0x1).is_loopback(), false);
     /// assert_eq!(Ipv6Addr::new(0, 0, 0, 0, 0, 0xffff, 0x7f00, 0x1).to_canonical().is_loopback(), true);
     /// ```
-    #[rustc_const_unstable(feature = "const_ipv6", issue = "76205")]
-    #[unstable(feature = "ip", issue = "27709")]
+    #[inline]
     #[must_use = "this returns the result of the operation, \
                   without modifying the original"]
-    #[inline]
+    #[stable(feature = "ip_to_canonical", since = "CURRENT_RUSTC_VERSION")]
+    #[rustc_const_stable(feature = "ip_to_canonical", since = "CURRENT_RUSTC_VERSION")]
     pub const fn to_canonical(&self) -> IpAddr {
         if let Some(mapped) = self.to_ipv4_mapped() {
             return IpAddr::V4(mapped);