Add Closable interface (#387)

src/Closable.php

@@ -0,0 +1,26 @@
+<?php
+
+namespace Amp;
+
+interface Closable
+{
+    /**
+     * Closes the resource, marking it as unusable.
+     * Whether pending operations are aborted or not is implementation dependent.
+     */
+    public function close(): void;
+
+    /**
+     * Returns whether this resource has been closed.
+     *
+     * @return bool {@code true} if closed, otherwise {@code false}
+     */
+    public function isClosed(): bool;
+
+    /**
+     * Registers a callback that is invoked when this resource is closed.
+     *
+     * @param \Closure():void $onClose
+     */
+    public function onClose(\Closure $onClose): void;
+}