Fuzz IP.Is<property>() methods against stdlib net.IP equivalents

The intent is to find discrepancies between netaddr.IP and net.IP. These
don't necessarily indicate a bug (in either), but it's better to find
them and explicitly document/exclude them.

fuzz.go

@@ -43,6 +43,29 @@ func Fuzz(b []byte) int {
 		panic(".Prior.Next did not round trip")
 	}
 
+	// Check that we agree with the standard library wrt to poperties
+	if !strings.Contains(s, "%") {
+		stdip := net.ParseIP(s)
+		tests := []struct {
+			name        string
+			ipResult    bool
+			stdipResult bool
+		}{
+			{"IsInterfaceLocalMulticast", ip.IsInterfaceLocalMulticast(), stdip.IsInterfaceLocalMulticast()},
+			{"IsLinkLocalMulticast", ip.IsLinkLocalMulticast(), stdip.IsLinkLocalMulticast()},
+			{"IsLinkLocalUnicast", ip.IsLinkLocalUnicast(), stdip.IsLinkLocalUnicast()},
+			{"IsLoopback", ip.IsLoopback(), stdip.IsLoopback()},
+			{"IsMulticast", ip.IsMulticast(), stdip.IsMulticast()},
+		}
+
+		for _, tt := range tests {
+			if tt.ipResult != tt.stdipResult {
+				fmt.Printf("net.IP=%#v .%v=%v, netaddr.IP=%#v %v=%v\n", stdip, tt.name, tt.stdipResult, ip, tt.name, tt.ipResult)
+				panic(fmt.Sprintf(".%v() did not agree with stdlib", tt.name))
+			}
+		}
+	}
+
 	port, err := ParseIPPort(s)
 	if err == nil {
 		checkStringParseRoundTrip(port, parseIPPort)