feat(probability/stopping): generalize `mem_ℒp_stopped_value` (#16369)
Also rename many lemmas which had names `*_of_encodable` but took `countable` arguments since a recent PR which replaced all `encodable` by `countable` but did not touch the names.
Co-authored-by: Rémy Degenne <remydegenne@gmail.com>
Co-authored-by: RemyDegenne <remydegenne@gmail.com>