De-dupe serialized objects, properly handle references

[grahamboree]

Currently circular references will cause infinite loops in serialization and de-serialization.

This self-referential tree structure...

class Test {
    public Test Left;
    public Test Right;
}

Test Root;
Root.Left = new Test();
Root.Right = Root;

...should be serialized as:

{
    "$id": 1,
    "Left": {
        "Left": null,
        "Right": null
    },
    "Right": {
        "$ref": 1
    }
}

$id gives a specific object in the json heirarchy an ID. These IDs should be unique per-document, but don't need to be a GUID. Smaller is better.

$ref indicates that the object containing the $ref key should be replaced by the object who's $id value is 1.

Additionally, this should be used to de-dupe objects that are serialized multiple times.

For example, the following...

class Test {
    public int Value;
    public double Amount;
    public string Name;
}

var data = new List<Test>();
var testData = new Test {
    Value = 42,
    Amount = 10.5,
    Name = "test name"
};
data.Add(testData);
data.Add(testData);
data.Add(testData);

...should be serialized as

[
    {
        "$id": 1,
        "Value": 42,
        "Amount": 10.5,
        "Name": "test name"
    },
    {
        "$ref": 1
    },
    {
        "$ref": 1
    }
]