Implement the Symbol builtin object (#2601)

This patch contains the base functionalities that the new builtin object requires.

Currently unavailable:
 - print (Symbol('foo')) - this features requires the refactor of the print handler function
 - Several global symbol based builtin routines (follow up patch)

JerryScript-DCO-1.0-Signed-off-by: Robert Fancsik frobert@inf.u-szeged.hu

jerry-core/ecma/operations/ecma-get-put-value.c

@@ -96,6 +96,7 @@ ecma_op_get_value_object_base (ecma_value_t base_value, /**< base value */
 
   JERRY_ASSERT (ecma_is_value_boolean (base_value)
                 || ecma_is_value_number (base_value)
+                || ECMA_ASSERT_VALUE_IS_SYMBOL (base_value)
                 || ecma_is_value_string (base_value));
 
   ecma_value_t object_base = ecma_op_to_object (base_value);