(And there have been various projects to provide some sub-capabilities for years.)
https://github.com/albertz/Pyjector (and then access the ObjC runtime via the Python ObjC bridge)
https://github.com/albertz/FScriptAnywhereSIMBL
http://www.cycript.org/ http://iphonedevwiki.net/index.php/Cycript
(And there have been various projects to provide some sub-capabilities for years.)