SI-8814  mutable.LongMap loses its LongMapness when adding with +

Added overrides for +, ++, and updated to both LongMap and AnyRefMap.

No tests; this was caught by scala-collections-laws.
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.
SI-8727  Map.filterKeys result's contains and get are inconsistent

Changed documentation and code so that predicate for filterKeys is always applied before the map is queried.
SI-8819  Range slice does not return a Range

Added an override to the `slice` method that creates a `Range`.  No tests (except verifying by hand that it solves the bug); scala-collections-laws found and will test this.
Merge pull request #4159 from Ichoran/issue/8727

SI-8727  Map.filterKeys result's contains and get are inconsistent
Merge pull request #4158 from Ichoran/issue/8765

SI-8765  immutable.HashSet exposes internal updated0 method
Merge pull request #4157 from Ichoran/issue/8814

SI-8814  mutable.LongMap loses its LongMapness when adding with +
Merge pull request #4156 from Ichoran/issue/8819

SI-8819  Range slice does not return a Range