[API] Improve the performance of the external magic id search (#1506)

After this patch, we have to provide external strings ordered by size and lexicographically.
We can do this with jerry_parse_and_save_literals() (#1500).

JerryScript-DCO-1.0-Signed-off-by: Zsolt Borbély zsborbely.u-szeged@partner.samsung.com

docs/02.API-REFERENCE.md

@@ -255,6 +255,8 @@ jerry_cleanup (void);
 
 Registers an external magic string array.
 
+*Note*: The strings in the array must be sorted by size at first, then lexicographically.
+
 **Prototype**
 
 ```c
@@ -275,11 +277,12 @@ jerry_register_magic_strings  (const jerry_char_ptr_t *ex_str_items_p,
   jerry_init (JERRY_INIT_EMPTY);
 
   // must be static, because 'jerry_register_magic_strings' does not copy
+  // the items must be sorted by size at first, then lexicographically
   static const jerry_char_ptr_t magic_string_items[] = {
                                                          (const jerry_char_ptr_t) "magicstring1",
                                                          (const jerry_char_ptr_t) "magicstring2",
                                                          (const jerry_char_ptr_t) "magicstring3"
-                                                        };
+                                                       };
   uint32_t num_magic_string_items = (uint32_t) (sizeof (magic_string_items) / sizeof (jerry_char_ptr_t));
 
   // must be static, because 'jerry_register_magic_strings' does not copy