Merge instance into context (#2501)

There was quite some confusion about terminology around instances
and contexts. All the docs mentioned external contexts but
functions and types were referring to instances, and the relation
between these two concepts were not clear. This commit keeps
(external) context as the only surviving concept.

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

jerry-core/include/jerryscript-port.h

@@ -147,17 +147,17 @@ bool jerry_port_get_time_zone (jerry_time_zone_t *tz_p);
 double jerry_port_get_current_time (void);
 
 /**
- * Get the current instance which contains the current context, heap and other
- * structures. Each port should provide its own implementation of this interface.
+ * Get the current context of the engine. Each port should provide its own
+ * implementation of this interface.
  *
  * Note:
  *      This port function is called by jerry-core when
  *      JERRY_ENABLE_EXTERNAL_CONTEXT is defined. Otherwise this function is not
  *      used.
  *
- * @return the pointer to the jerry instance.
+ * @return the pointer to the engine context.
  */
-struct jerry_instance_t *jerry_port_get_current_instance (void);
+struct jerry_context_t *jerry_port_get_current_context (void);
 
 /**
  * Makes the process sleep for a given time.