Add Input::pointers() to iterate over JSON pointers

[thekid]

This pull request adds pointers() complementing the pairs() and elements() iterations. This makes it possible to stream values even when the data contains a wrapper, such as the data key often seen in REST APIs.

Example

Input data:

{
  "data": [
    "string 1",
    "string 2",
    "string 3",
    // ...
  ]
}

Processing:

use io\streams\FileInputStream;
use text\json\StreamInput;

$input= new StreamInput(new FileInputStream($argv[1]));
foreach ($input->pointers() as $pointer => $value) {
  if (str_starts_with($pointer, '/data/')) {
    echo $pointer, ' => ', $value, "\n";
  }
}