It seems that `cffi:defcallback` doesn't currently use libffi for creation callbacks.
E.g. in case of sbcl, it calls `%defcallback`, that is defined in `cffi-sbcl.lisp` and calls SBCL's own `alien-sap`.
That means, if we want to add support for libffi callbacks, we need to define a new macro, like `cffi:defcallback-libffi` or something.
It seems that `cffi:defcallback` doesn't currently use libffi for creation callbacks.
E.g. in case of sbcl, it calls `%defcallback`, that is defined in `cffi-sbcl.lisp` and calls SBCL's own `alien-sap`.
That means, if we want to add support for libffi callbacks, we need to define a new macro, like `cffi:defcallba ck-libffi` or something.