This was needed especially for concurrent fold combinators. A fold
combinator that uses concurrent folds needs to wait for the concurrent
folds to finish before it can finish. The finalizing action in folds can
deallocate any resources allocated by the "initial" action and also wait
for folds that it has initialized.
This complicates fold combinators in general. We can potentially
introduce a type for non-failing parsers and support finalization only
in those. The current use cases can be covered by that. Parsers do not
support scanning, which is not required in the use cases where we need
finalization (there is no known use case).
