Use native functions

Features:
  * use native json_encode() and json_decode() if they are available
  * use native memory_get_usage()
  * use native zend_zval_type_name() instead of userspace gettype()
  * use native zend_fetch_debug_backtrace() instead of debug_backtrace()
  * use native counterparts for header() and headers_sent()
  * use native session_start() and session_id()
  * use native ob_get_level()
  * use native lcfirst()
  * updated gen-build.php and rebuilt release files