Iterator support for Arrays & Objects

[80avin]

I am really impressed by this project but I find the current API less helpful.

Hence, proposing a new way to iterate over the array elements or object entries.

Example:

data := []bytes(`{"menu": {
    "header": "SVG Viewer",
    "items": [
        {"id": "Open"},
        {"id": "OpenNew", "label": "Open New"},
        null,
        {"id": "ZoomIn", "label": "Zoom In"},
        {"id": "ZoomOut", "label": "Zoom Out"},
        {"id": "OriginalView", "label": "Original View"},
        null,
        {"id": "Quality"},
        {"id": "Pause"},
        {"id": "Mute"},
        null,
        {"id": "Find", "label": "Find..."},
        {"id": "FindAgain", "label": "Find Again"},
        {"id": "Copy"},
        {"id": "CopyAgain", "label": "Copy Again"},
        {"id": "CopySVG", "label": "Copy SVG"},
        {"id": "ViewSVG", "label": "View SVG"},
        {"id": "ViewSource", "label": "View Source"},
        {"id": "SaveAs", "label": "Save As"},
        null,
        {"id": "Help"},
        {"id": "About", "label": "About Adobe CVG Viewer..."}
    ]
}}`)

// Either support object notation
iterator, err := jsonparser.ArrayIterator(data, "menu", "items")
for val, err = iterator.next(); err == nil && iterator.hasNext(); val, err = iterator.next() {
    // iterator.hasNext() can be replaced by err = errors.New("Done")
    // do anything or break as required.
  }

// OR support closure notation
next(), hasNext, err = jsonparser.ArrayIterator(data, "menu", "items")
for val, hasNext, err = next(); err == nil && hasNext; val, hasNext, err = next() {
  // hasNext can be dropped as err = errors.New("Done") can convey same information.
  // do anything or break as required.
}

Similar syntax can be used for iterating object entries (key/values), or other functions like EachKey which ask for a callback to iterate over values.

This will prevent the callback hell and also make the code more efficient & readable since we need not iterate over all entries to find one.

PS. I can also contribute and create a PR once it is approved.