feat: the Stringer implements in JSON mode

[tyrzhang]

feat: the Stringer implements in JSON mode

output string example:

{"size":1,"depth":3,"root":{"entries":[{"bb":"[1.00, 2.00]x[1.00, 2.00]","child":{"entries":[{"bb":"[1.00, 2.00]x[1.00, 2.00]","child":{"leaf":true,"entries":[{"bb":"[1.00, 2.00]x[1.00, 2.00]"}]}}]}}]}}

format the example

{
  "size": 1,
  "depth": 3,
  "root": {
    "entries": [
      {
        "bb": "[1.00, 2.00]x[1.00, 2.00]",
        "child": {
          "entries": [
            {
              "bb": "[1.00, 2.00]x[1.00, 2.00]",
              "child": {
                "leaf": true,
                "entries": [
                  {
                    "bb": "[1.00, 2.00]x[1.00, 2.00]"
                  }
                ]
              }
            }
          ]
        }
      }
    ]
  }
}

[dhconnelly]

People will start relying on the fact that the string representation is JSON. (Conway's Law :)) Could you add a test case to verify it will parse properly? A small but nontrivial tree should be sufficient.