use iodata for json serialization

[dkuku]

@josevalim New pr because there is too much changes comparing to the previous one
The previous one can still be used after making the new functions private.
In the new test case for encode_to_iodata! the list currently looks like this:

[
  91,
  [34, [["12", 58, "34", 58 | "56"], 46 | "78"], 34],
  44,
  [34, ["2024", 45, "12", 45 | "31"], 34],
  44,
  [
    34,
    ["2010", 45, ["0" | "4"], 45 | "17"],
    84,
    [["14", 58, ["0" | "0"], 58, "0" | "0"], 46 | "123"],
    34
  ],
  44,
  [
    34,
    ["2010", 45, ["0" | "4"], 45 | "17"],
    84,
    [["14", 58, ["0" | "0"], 58, "0" | "0"], 46 | "123"],
    90,
    34
  ],
  44,
  [34, "P2MT3H", 34],
  93
]

[josevalim]

💚 💙 💜 💛 ❤️