Fix #46

This is the bare minimum required to make fibers work within the go
  runtime.

testdata/fiber-basic.php

@@ -0,0 +1,9 @@
+<?php
+require_once __DIR__.'/_executor.php';
+
+return function() {
+    $fiber = new Fiber(function() {
+        echo 'Fiber '.($_GET['i'] ?? '');
+    });
+    $fiber->start();
+};