[girffi] Clean up API, add g_function_info_prep_invoker

[girffi] Clean up API, add g_function_info_prep_invoker

Rather than having bindings use g_function_info_invoke, which is basically
a toy/demo API, export a convenience utility function which takes the introspection
information and sets up things we need to pass to libffi.

Then invocation can be done directly to libffi by a binding.

As part of this work, remove some (unused by gjs) public functions from the
girffi API, and instead export a function to map to libffi which can work
semi-correctly.

https://bugzilla.gnome.org/show_bug.cgi?id=604074