Implement common external function handlers in jerry-ext (#1787)

Added `handler` module to `jerry-ext` to contain implementation of
commonly used external function handlers: `assert`, `gc`, and
`print`.

Also adapted jerry-main to use jerry-ext/handler

JerryScript-DCO-1.0-Signed-off-by: Akos Kiss akiss@inf.u-szeged.hu

jerry-port/default/defaultx-handler.c

@@ -0,0 +1,28 @@
+/* Copyright JS Foundation and other contributors, http://js.foundation
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <stdio.h>
+
+#include "jerryscript-ext/handler.h"
+
+/**
+ * Default implementation of jerryx_port_handler_print_char. Uses 'printf' to
+ * print a single character to standard output.
+ */
+void
+jerryx_port_handler_print_char (char c) /**< the character to print */
+{
+  printf ("%c", c);
+} /* jerryx_port_handler_print_char */