Extending support for negative string offsets

Hi, I am starting a PR (https://github.com/php/php-src/pull/1431) to add support for negative string offsets (counting from the end of the container string) where they could and, IMHO, should be supported. The already-published commits add support for negative offsets in '$string{$offset}' expressions (read and write), something that had been on my todo list for a long time. This one especially needs review because I'm not so familiar with the engine. Next changes will add support for functions which don't support negative offsets yet (strpos, stripos,...). Support for negative lengths may be added too. Please give thoughts. If return is positive, I'll write an RFC targeting 7.1. Regards François