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
SI-7981  toSeq on immutable Map and Set return ArrayBuffer

Switched to `Vector` for the default `MapLike`/`SetLike`, then back again in `mutable.MapLike`/`mutable.SetLike`.

Preliminary performance testing indicates an across-the-board improvement due to cutting out a great deal of indirection.  In the case of empty immutable maps or sets, the improvement is enormous: it quickly returns Vector.empty instead of jumping through all sorts of hoops to eventually create an empty `ArrayBuffer`.

One possible downside is that `Vector` is pretty memory-hungry for small collections.  Need to evalute this; rather than abandoning this change, it's probably better to make `Vector` smarter about small collections.

Also fixed test with literal "ArrayBuffer" in output (`s/ArrayBuffer/Vector/g`).
Merge pull request #4175 from Ichoran/issue/7981

SI-7981  toSeq on immutable Map and Set return ArrayBuffer
SI-8691  SeqView throws exception when prepending a collection

Prepend was throwing segfaults as it was handled differently from Append.  This brings the two into line with each other.

There are various optimizations that could be applied that have not been, however, such as intercepting sequential prepends and generating one multi-prepend instead of nested single-element prepends.

Unit test added to verify minimally that bug is gone.