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/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
+}