[RFC] Fiber support (again)

Hi internals, I propose to introduce the Fiber feature AGAIN. The main purpose of the RFC is to introducing a lightweight stackful coroutine support for PHP and make it possible to write non-blocking code in the blocking style. In this RFC, no new keyword is needed. So it will not break the PHP 7.3 release. Please see the RFC https://wiki.php.net/rfc/fiber Dmitry and I are working on the implementation at https://github.com/fiberphp/fiber-ext And a series of usage demo can be found at https://github.com/fiberphp/fiber-demo Please offer you comments. Thank you. --- Haitao Lv