Make go module, marshal userdata as json(null), and encode sparse arrays

[mikefero]

To improve the gopher-json encoding aspects lua.UserData types detected are replaced as JSON null values and the encoding of sparse arrays is now possible where the key/index and value from the array item are combined to form the JSON value:

{
  1,
  2,
  [10] = 3
}

=>

[
  1,
  2,
  "[10] = 3"
]