SI-8765  immutable.HashSet exposes internal updated0 method

Made most internal methods protected or private[immutable], as appropriate.  (Coverage was scattershot before.)

updated0 is private[collection] since it is used by ParHashSet.  (Not sure how wise this is for maintainability, but let's not rock this boat now.)

No deprecation, as there was no real way to sanely use these.  No test, as this is better caught by inspection than compilation failure tests.