varlinkgen: allow generated handlers to reply asynchronously

• Should the generated reply functions take ownership over the out-params and free them? Or should we make all struct members const?
• The type system doesn't enforce that the right out-params type is sent to the right call. Given two methods Foo and Bar, it's possible to reply Foo_out to Bar. Should we generate a wrapper type for varlinkgen_service_call to enforce this?