Enhance operation on untyped json through operator overload based on json pointer

[lymslive]

Although it is wonderfull to deserilize json to strong type struct, sometimes it has to deal with untyped json, especially when the input json is too loose or too freely to fit a struct hierarchy. So I devolop a solution for such case.

I summarize the following basic operation on untyped json tree:

1. Locate and refer to some node in the tree by json pointer syntax, using operator / overload.
2. Read value from leaf node with default fallback, using operator |, image as get_or method.
3. Write value to json node as container of common data type, using operator <<, image as push item to array or object, or overwrite put value to scalar node.
4. Some advanced modification on json tree as a whole, using operator |, pipe to function or closure.

All the operators above can be chained as return json pointer, except case 2 which only return primitive scalar value.

Example from that added to README.md:

let v: Value = json( {
        "name": "John Doe",
        "age": 43,
        "phones": [
            "+44 1234567",
            "+44 2345678"
        ]
    });

let age_ptr = &v / "age";
let age_val = age_ptr | 0; // got 43, or default 0 if something wrong.
let first_phone = &v / "phones" / 0 | ""; // got &str
assert_eq!(first_phone, "+44 1234567");

// modify some node
let _ = &mut v / "age" << 44;
let _ = &mut v / "phones" << ["+44 3456789"];
let _ = &mut v << ("sex": "male");

And there is also fully documantation example and tests.

Because it is impossible to overload operator for serde_json::Value outside the crate, but at most can only to overload operator for new struct for json pointer. So I create this pull request to support json operator natively as well.

[dtolnay]

Thank you for the PR, but I would prefer not to support this mechanism in this library.