Make Ipv{4,6}Addr::new const fns

src/libstd/lib.rs

@@ -252,6 +252,7 @@
 #![feature(char_error_internals)]
 #![feature(compiler_builtins_lib)]
 #![feature(const_fn)]
+#![feature(const_int_ops)]
 #![feature(core_intrinsics)]
 #![feature(dropck_eyepatch)]
 #![feature(exact_size_is_empty)]
@@ -281,6 +282,7 @@
 #![feature(ptr_internals)]
 #![feature(raw)]
 #![feature(rustc_attrs)]
+#![feature(rustc_const_unstable)]
 #![feature(std_internals)]
 #![feature(stdsimd)]
 #![feature(shrink_to)]