Stabilize IpAddr::is_ipv4 and is_ipv6 as const

Insta-stabilize the methods `is_ipv4` and `is_ipv6` of `IpAddr`.

Possible because of the recent stabilization of const control flow.

Also adds a test for these methods in a const context.

src/test/ui/consts/std/net/ip.rs

@@ -0,0 +1,13 @@
+// run-pass
+
+use std::net::{IpAddr, Ipv4Addr};
+
+fn main() {
+    const IP_ADDRESS : IpAddr = IpAddr::V4(Ipv4Addr::LOCALHOST);
+
+    const IS_IP_V4 : bool = IP_ADDRESS.is_ipv4();
+    assert!(IS_IP_V4);
+
+    const IS_IP_V6 : bool = IP_ADDRESS.is_ipv6();
+    assert!(!IS_IP_V6);
+}