Q: how to access IPv6 ext headers?

Hi,
since gopacket doesn't provide any IP fragmentation support, I want to write one, however after reading the layers.IPv6 doc, I can't figure out how gopacket handle the IPv6 ext headers, there is only a HopByHop field in the struct, how does user access other v6 ext header like routing header after using gopacket.NewPacket() ?

```
type IPv6 struct {
// http://www.networksorcery.com/enp/protocol/ipv6.htm
BaseLayer
Version uint8
TrafficClass uint8
FlowLabel uint32
Length uint16
NextHeader IPProtocol
HopLimit uint8
SrcIP net.IP
DstIP net.IP
HopByHop *IPv6HopByHop
// contains filtered or unexported fields
}
```