Return JSON objects in a wrapper object

[thekid]

This pull request changes JSON objects to be returned as an instance of text.json.JsonObject instead of as an associative array.

In a nutshell

use text\json\Json;

$empty= Json::read('{}');                // JsonObject([])
$value= Json::read('{"key" : "value"}'); // JsonObject(['key' => 'value'])

This makes us able to distinguish {} from [], which previously would have both yielded an empty array:

use text\json\Json;

$roundtrip= Json::of(Json::read('{}'));  // Before: '[]' (❌), After: '{}' (✅)

Decision to create dedicated class

We could have used StdClass like json_decode() does but would break BC as array access does not work, yielding Cannot use object of type stdClass as array.

use text\json\Json;

$value= Json::read('{"color" : "green"}');

// All of the following would have been broken, throwing exceptions 💣
isset($value['color']);
unset($value['color']);
$color= $value['color'];
$n= sizeof($value);

There is, however, still some BC breaks. If you pass the returned value to a function with an array type hint, use the + operator for a union, use is_array(), any of the array functions or rely on an assignment creating a copy, or if you use empty(), the behavior will be different!

[thekid]

This makes us able to distinguish {} from [], which previously would have both yielded an empty array:

See #16

[thekid]

Closed in favor of #16