Still TODO in 2.x development

[jcupitt]

The FFI version is merged to master, the obvious remaining TODOs are:

• [ ] Support preloading, see https://www.php.net/manual/en/class.ffi.php
• [ ] Rewrite the enum and doc generator in php.
• [ ] Add source/target API
• [ ] Add progress callbacks etc.
• [ ] Add mutable.