node.go
func (uuid UUID) NodeID() []byte {
	var node [6]byte
	copy(node[:], uuid[10:])
	return node[:]
}
func (uuid UUID) NodeID() []byte {
	var node [6]byte
	copy(node[:], uuid[10:])
	return node[:]
}
// NodeID returns the 6 byte node id encoded in uuid.  It returns nil if uuid is
// not valid.  The NodeID is only well defined for version 1 and 2 UUIDs.