Install psalm as dev, sync scripts updates

vendor/amphp/amp/lib/CancelledException.php

@@ -0,0 +1,17 @@
+<?php
+
+namespace Amp;
+
+/**
+ * Will be thrown in case an operation is cancelled.
+ *
+ * @see CancellationToken
+ * @see CancellationTokenSource
+ */
+class CancelledException extends \Exception
+{
+    public function __construct(\Throwable $previous = null)
+    {
+        parent::__construct("The operation was cancelled", 0, $previous);
+    }
+}