net/packet: support full IPv6 decoding.

The packet filter still rejects all IPv6, but decodes enough from v6
packets to do something smarter in a followup.

name              time/op
Decode/tcp4-8     28.8ns ± 2%
Decode/tcp6-8     20.6ns ± 1%
Decode/udp4-8     28.2ns ± 1%
Decode/udp6-8     20.0ns ± 6%
Decode/icmp4-8    21.7ns ± 2%
Decode/icmp6-8    14.1ns ± 2%
Decode/unknown-8  9.43ns ± 2%

Signed-off-by: David Anderson <danderson@tailscale.com>

net/packet/icmp6.go

@@ -0,0 +1,37 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package packet
+
+type ICMP6Type uint8
+
+const (
+	ICMP6Unreachable  ICMP6Type = 1
+	ICMP6TimeExceeded ICMP6Type = 3
+	ICMP6EchoRequest  ICMP6Type = 128
+	ICMP6EchoReply    ICMP6Type = 129
+)
+
+func (t ICMP6Type) String() string {
+	switch t {
+	case ICMP6Unreachable:
+		return "Unreachable"
+	case ICMP6TimeExceeded:
+		return "TimeExceeded"
+	case ICMP6EchoRequest:
+		return "EchoRequest"
+	case ICMP6EchoReply:
+		return "EchoReply"
+	default:
+		return "Unknown"
+	}
+}
+
+type ICMP6Code uint8
+
+const (
+	ICMP6NoCode ICMP6Code = 0
+)
+
+const icmp6HeaderLength = 4