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`.
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