Lay out device/stack interfaces - Netdev/Netlink (#92)

* good code today

* add netdev.Runner

* round off APIs more

* better interface method documentation

* improve DHCP netstack API

* add pico w netdev example

* remove temp file

* keep thinking about this. this is hard :/

* fix ci

* small fixer

* working dhcp

* fix rebase API mismatches

* begin adding espradio example

* keep working on espradio, icmp not working

* fix icmp by fixing dhcp

x/xnet/xnet_debug_pcap.go

@@ -0,0 +1,17 @@
+//go:build xnetdebug
+
+package xnet
+
+import "os"
+
+var _pcap CapturePrinter
+
+func init() {
+	_pcap.Configure(os.Stdout, CapturePrinterConfig{
+		NamespaceWidth: 3,
+	})
+}
+
+func debugPacket(msg string, b []byte) {
+	_pcap.PrintPacket(msg, b)
+}