Data JSON support

[johanjanssens]

This PR adds several improvements to the data objects.

1. JSON support

It's now possible to cast a data object to a string. You can do so by calling the data([..])->toString() method, or you can cast the the data object to a string <?= data([...]) ?>

2. Create a data object from an array

It's now possible to create a new data object from an associative array of data. Example

<? $data = data([
    'foo' => 'bar',
    'dog' => 'bark'
]) ?>

A none associative array is not supported. This will be interpreted as an array of data locations, see also #4