Create PHP version (same repository)

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For the Rarch format to be viable across all our use cases, a PHP 5.5+ (if not 7+) version is needed. This implies finding the PHP equivalent of "vcdiff" which may not be trivial, and we might need to roll our own in both languages to control this tightly.

Thanks to the positive files{} section of package.json and the negative archive.exclude{} section of composer.json, there is no need to create two separate repositories for JavaScript and PHP. (It would be interesting to see if this could be continued for other languages as well.) So let's just move rarch.js and utils.js to a new subdirectory js/ and create php/ for the PHP class.