all: update to Go 1.18

Bump go.mod and README.

Switch to upstream net/netip.

Use strings.Cut.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

conn/bind_linux.go

@@ -8,13 +8,13 @@ package conn
 import (
 	"errors"
 	"net"
+	"net/netip"
 	"strconv"
 	"sync"
 	"syscall"
 	"unsafe"
 
 	"golang.org/x/sys/unix"
-	"golang.zx2c4.com/go118/netip"
 )
 
 type ipv4Source struct {

conn/bind_std.go

@@ -8,10 +8,9 @@ package conn
 import (
 	"errors"
 	"net"
+	"net/netip"
 	"sync"
 	"syscall"
-
-	"golang.zx2c4.com/go118/netip"
 )
 
 // StdNetBind is meant to be a temporary solution on platforms for which

conn/bind_windows.go

@@ -9,13 +9,13 @@ import (
 	"encoding/binary"
 	"io"
 	"net"
+	"net/netip"
 	"strconv"
 	"sync"
 	"sync/atomic"
 	"unsafe"
 
 	"golang.org/x/sys/windows"
-	"golang.zx2c4.com/go118/netip"
 
 	"golang.zx2c4.com/wireguard/conn/winrio"
 )

conn/bindtest/bindtest.go

@@ -9,9 +9,9 @@ import (
 	"fmt"
 	"math/rand"
 	"net"
+	"net/netip"
 	"os"
 
-	"golang.zx2c4.com/go118/netip"
 	"golang.zx2c4.com/wireguard/conn"
 )
 

conn/conn.go

@@ -9,11 +9,10 @@ package conn
 import (
 	"errors"
 	"fmt"
+	"net/netip"
 	"reflect"
 	"runtime"
 	"strings"
-
-	"golang.zx2c4.com/go118/netip"
 )
 
 // A ReceiveFunc receives a single inbound packet from the network.