add tcpip stack framework

Add basis for TCP/IP stack.  The stack implements Netdever interface for
the top-end, and calls into a Netlinker interface on the bottom-end.
Each TCP/IP stack instance represents a L3/L4 endpoint with an IP
address.  The Netlinker is bound when creating the stack.  E.g.:

	spi, cs, wlreg, irq := cyw43439.PicoWSpi(0)
	cyw43 := cyw43439.NewDevice(spi, cs, wlreg, irq, irq)
	stack := tcpip.NewStack(cyw43)
	netdev.UseNetdev(stack)

Here, the cyw43439 driver is the Netlinker for the stack.  The new stack
is a Netdever, so we tell the "net" package to use the stack as the
netdev.  The stack manages L3/L4 socket connections, ultimately calling
into the Netlinker to send/recv L2 Ethernet pkts.

tcpip/netdev.go

@@ -0,0 +1,52 @@
+package tcpip
+
+import (
+	"net"
+	"time"
+
+	"tinygo.org/x/drivers/netdev"
+)
+
+func (s *Stack) GetHostByName(name string) (net.IP, error) {
+	return net.IP{}, netdev.ErrNotSupported
+}
+
+func (s *Stack) GetIPAddr() (net.IP, error) {
+	return net.IP{}, netdev.ErrNotSupported
+}
+
+func (s *Stack) Socket(domain int, stype int, protocol int) (int, error) {
+	return -1, netdev.ErrNotSupported
+}
+
+func (s *Stack) Bind(sockfd int, ip net.IP, port int) error {
+	return netdev.ErrNotSupported
+}
+
+func (s *Stack) Connect(sockfd int, host string, ip net.IP, port int) error {
+	return netdev.ErrNotSupported
+}
+
+func (s *Stack) Listen(sockfd int, backlog int) error {
+	return netdev.ErrNotSupported
+}
+
+func (s *Stack) Accept(sockfd int, ip net.IP, port int) (int, error) {
+	return -1, netdev.ErrNotSupported
+}
+
+func (s *Stack) Send(sockfd int, buf []byte, flags int, deadline time.Time) (int, error) {
+	return 0, netdev.ErrNotSupported
+}
+
+func (s *Stack) Recv(sockfd int, buf []byte, flags int, deadline time.Time) (int, error) {
+	return 0, netdev.ErrNotSupported
+}
+
+func (s *Stack) Close(sockfd int) error {
+	return netdev.ErrNotSupported
+}
+
+func (s *Stack) SetSockOpt(sockfd int, level int, opt int, value interface{}) error {
+	return netdev.ErrNotSupported
+}

tcpip/stack.go

@@ -0,0 +1,20 @@
+package tcpip
+
+import (
+	"tinygo.org/x/drivers/netlink"
+)
+
+type Stack struct {
+	link netlink.Netlinker
+}
+
+func NewStack(link netlink.Netlinker) *Stack {
+	s := Stack{link: link}
+	s.link.RecvEthFunc(s.recvEth)
+	return &s
+}
+
+func (s *Stack) recvEth(pkt []byte) error {
+	println("recvEth", len(pkt))
+	return nil
+}