Faster phalcon_array_merge_recursive_n()

Changes:
  * dropped `array_merge_recursive_n()` because it was a duplicate of `phalcon_array_merge_recursive_n()`
  * introduced `phalcon_get_current_key_w()` that returns a `zval` that boxes the current key; this zval does not have to be destroyed, therefore PHALCON_MM_GROW()/PHALCON_MM_RESTORE() can be sometimes avoided;
  * removed `TSRMLS` from `phalcon_array_merge_recursive_n()` thanks to `phalcon_get_current_key_w()`