Add a const_sockaddr_setters feature

Unstably add `const` to the `sockaddr_setters` methods. Included API:

    // core::net

    impl SocketAddr {
        pub const fn set_ip(&mut self, new_ip: IpAddr);
        pub const fn set_port(&mut self, new_port: u16);
    }

    impl SocketAddrV4 {
        pub const fn set_ip(&mut self, new_ip: Ipv4Addr);
        pub const fn set_port(&mut self, new_port: u16);
    }

    impl SocketAddrV6 {
        pub const fn set_ip(&mut self, new_ip: Ipv6Addr);
        pub const fn set_port(&mut self, new_port: u16);
    }

Tracking issue: <https://github.com/rust-lang/rust/issues/131714>

library/core/src/lib.rs

@@ -136,6 +136,7 @@
 #![feature(const_raw_ptr_comparison)]
 #![feature(const_size_of_val)]
 #![feature(const_size_of_val_raw)]
+#![feature(const_sockaddr_setters)]
 #![feature(const_strict_overflow_ops)]
 #![feature(const_swap)]
 #![feature(const_try)]