layers.init cost too much time

layers.init cost about 0.2s to init.

Make it lazy init or manually init should be good.
Make it faster is also an option.
But this kind of change will break the exist clients.

* https://github.com/google/gopacket/blob/master/layers/iana_ports.go#L18 here use 176KB heap memory.
* https://github.com/google/gopacket/blob/master/layers/iana_ports.go#L5817 here use another 176KB heap memory.