Use native functions

[ghost]

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

[phalcon]

Thank you!!