Now the model has a mime_type that we expect to return to the UI, and
model_name so the name of the model on disk can be impl-specific.
Moreover, now Model and Container use the same logic to detect if
a mimetype is textual.
The `serialize` function of `revng::pypeline::Model` erroneously
returned a buffer instead of `bytes`, fix the function so that `bytes`
are returned instead.