Rework which information is transmitted in the pipeline metadata,
avoiding redundancy and moving some information there instead of
returning it every time a request is made.
Overhaul the logic and method names involved in enabling and disabling
reference caching in `TupleTree<T>`. `TupleTreeReference<T, U>` now
lazily caches the target and will traverse the path only when needed.
Also expose and use these functions in the new pipeline, which should
provide some speedup when executing a `Schedule`.
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.
Giacomo Vercesi