Migrate to new register / unregister API for TaskScheduler

composer.json

@@ -2,6 +2,8 @@
     "name": "amphp/amp",
     "homepage": "http://amphp.org/amp",
     "description": "A non-blocking concurrency framework for PHP applications.",
+    "minimum-stability": "dev",
+    "prefer-stable": true,
     "keywords": [
         "async",
         "asynchronous",

lib/Internal/functions.php

@@ -4,7 +4,7 @@ namespace Amp\Internal;
 
 use Concurrent\TaskScheduler;
 
-TaskScheduler::setDefaultScheduler($GLOBALS["__amp_scheduler"] = new Scheduler);
+TaskScheduler::register(new Scheduler);
 
 /**
  * Creates a `TypeError` with a standardized error message.