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

docs/10.EXT-REFERENCE-HANDLER.md

@@ -0,0 +1,179 @@
+#  Common external function handlers
+
+## jerryx_handler_assert
+
+**Summary**
+
+Assert for scripts. The routine calls `jerry_port_fatal` on assertion failure.
+
+**Prototype**
+
+```c
+jerry_value_t
+jerryx_handler_assert (const jerry_value_t func_obj_val, const jerry_value_t this_p,
+                       const jerry_value_t args_p[], const jerry_length_t args_cnt);
+```
+
+- `func_obj_val` - the function object that was called (unused).
+- `this_p` - the `this` value of the call (unused).
+- `args_p` - the array of function arguments.
+- `args_cnt` - the number of function arguments.
+- return value - `jerry_value_t` representing boolean true, if only one argument
+  was passed and that argument was a boolean true. Note that the function does
+  not return otherwise.
+
+**See also**
+
+- [jerryx_handler_register_global](#jerryx_handler_register_global)
+
+
+## jerryx_handler_gc
+
+**Summary**
+
+Expose garbage collector to scripts.
+
+**Prototype**
+
+```c
+jerry_value_t
+jerryx_handler_gc (const jerry_value_t func_obj_val, const jerry_value_t this_p,
+                   const jerry_value_t args_p[], const jerry_length_t args_cnt);
+```
+
+- `func_obj_val` - the function object that was called (unused).
+- `this_p` - the `this` value of the call (unused).
+- `args_p` - the array of function arguments (unused).
+- `args_cnt` - the number of function arguments (unused).
+- return value - `jerry_value_t` representing `undefined`.
+
+**See also**
+
+- [jerryx_handler_register_global](#jerryx_handler_register_global)
+
+
+## jerryx_handler_print
+
+**Summary**
+
+Provide a `print` implementation for scripts. The routine converts all of its
+arguments to strings and outputs them char-by-char using
+`jerryx_port_handler_print_char`. The NUL character is output as "\u0000",
+other characters are output bytewise.
+
+*Note*: This implementation does not use standard C `printf` to print its
+output. This allows more flexibility but also extends the core JerryScript
+engine port API. Applications that want to use `jerryx_handler_print` must
+ensure that their port implementation also provides
+`jerryx_port_handler_print_char`.
+
+**Prototype**
+
+```c
+jerry_value_t
+jerryx_handler_print (const jerry_value_t func_obj_val, const jerry_value_t this_p,
+                      const jerry_value_t args_p[], const jerry_length_t args_cnt);
+```
+
+- `func_obj_val` - the function object that was called (unused).
+- `this_p` - the `this` value of the call (unused).
+- `args_p` - the array of function arguments.
+- `args_cnt` - the number of function arguments.
+- return value - `jerry_value_t` representing `undefined` if all arguments could
+  be converted to strings, an `Error` otherwise.
+
+**See also**
+
+- [jerryx_handler_register_global](#jerryx_handler_register_global)
+- [jerryx_port_handler_print_char](#jerryx_port_handler_print_char)
+
+
+# Handler registration helper
+
+## jerryx_handler_register_global
+
+**Summary**
+
+Register a JavaScript function in the global object.
+
+*Note*: Returned value must be freed with `jerry_release_value`, when it is no
+longer needed.
+
+**Prototype**
+
+```c
+jerry_value_t
+jerryx_handler_register_global (const jerry_char_t *name_p,
+                                jerry_external_handler_t handler_p);
+```
+
+- `name_p` - the name of the function to be registered.
+- `handler_p` - the address of the external function handler.
+- return value - `jerry_value_t` representing boolean true, if the operation was
+  successful, an `Error` otherwise.
+
+**Example**
+
+```c
+#include "jerryscript.h"
+#include "jerryscript-ext/handler.h"
+
+static const struct {
+  const char *name_p;
+  jerry_external_handler_t handler_p;
+} common_functions[] =
+{
+  { "assert", jerryx_handler_assert },
+  { "gc", jerryx_handler_gc },
+  { "print", jerryx_handler_print },
+  { NULL, NULL }
+};
+
+static void register_common_functions ()
+{
+  jerry_value_t ret = jerry_create_undefined ();
+
+  for (int i = 0; common_functions[i].name_p != NULL && !jerry_value_has_error_flag (ret); i++)
+  {
+    ret = jerryx_handler_register_global ((const jerry_char_t *) common_functions[i].name_p,
+                                          common_functions[i].handler_p);
+  }
+
+  return ret;
+}
+```
+
+
+# Port API extension
+
+## jerryx_port_handler_print_char
+
+**Summary**
+
+Print a single character.
+
+**Prototype**
+
+```c
+void
+jerryx_port_handler_print_char (char c);
+```
+
+- `c` - the character to print.
+
+**Example**
+
+```c
+/**
+ * Print a character to stdout with printf.
+ */
+void
+jerryx_port_handler_print_char (char c)
+{
+  printf ("%c", c);
+} /* jerryx_port_handler_print_char */
+```
+
+**See also**
+
+- [jerryx_handler_print](#jerryx_handler_print)