TreeMap is serialization compat with 2.12.10 again

Explicitly lock down the SerialVersionUID and avoid having
scalac mangle the name of the `tree` field to make it non-private.

Reinstate serialization compatibility tests for this and other
collections that were temporarily disabled due to an interation
with -Xcheckinit. Now, we simply skip the test when we detect
that the build is using that option.

build.sbt

@@ -270,6 +270,7 @@ val mimaFilterSettings = Seq {
     ProblemFilters.exclude[DirectMissingMethodProblem]("scala.collection.immutable.Map#Map4.foreachEntry"),
     ProblemFilters.exclude[MissingTypesProblem]("scala.collection.immutable.ListMap"),
     ProblemFilters.exclude[MissingTypesProblem]("scala.collection.immutable.TreeMap"),
+    ProblemFilters.exclude[MissingFieldProblem]("scala.collection.immutable.TreeMap.serialVersionUID"),
     ProblemFilters.exclude[MissingTypesProblem]("scala.collection.immutable.HashMap"),
     ProblemFilters.exclude[MissingTypesProblem]("scala.collection.immutable.HashMap$HashMap1"),
     ProblemFilters.exclude[MissingTypesProblem]("scala.collection.immutable.HashMap$HashTrieMap"),

src/library/scala/collection/immutable/TreeMap.scala

@@ -48,12 +48,16 @@ object TreeMap extends ImmutableSortedMapFactory[TreeMap] {
  *  @define mayNotTerminateInf
  *  @define willNotTerminateInf
  */
+@SerialVersionUID(4714724050750123970L)
 final class TreeMap[A, +B] private (tree: RB.Tree[A, B])(implicit val ordering: Ordering[A])
   extends SortedMap[A, B]
      with SortedMapLike[A, B, TreeMap[A, B]]
      with MapLike[A, B, TreeMap[A, B]]
      with Serializable
      with HasForeachEntry[A, B] {
+  // Manually use this from inner classes to avoid having scalac rename `tree` to an expanded name which is not
+  // serialization compatbile.
+  private def tree0: RB.Tree[A, B] = tree
 
   override protected[this] def newBuilder : Builder[(A, B), TreeMap[A, B]] =
     TreeMap.newBuilder[A, B]
@@ -224,10 +228,10 @@ final class TreeMap[A, +B] private (tree: RB.Tree[A, B])(implicit val ordering:
   }
 
   override def keySet: SortedSet[A] = new DefaultKeySortedSet {
-    override def foreach[U](f: A => U): Unit = RB.foreachEntry(tree, {(key: A, _: B) => f(key)})
+    override def foreach[U](f: A => U): Unit = RB.foreachEntry(tree0, {(key: A, _: B) => f(key)})
   }
 
   override def values: scala.Iterable[B] = new DefaultValuesIterable {
-    override def foreach[U](f: B => U): Unit = RB.foreachEntry(tree, {(_: A, value: B) => f(value)})
+    override def foreach[U](f: B => U): Unit = RB.foreachEntry(tree0, {(_: A, value: B) => f(value)})
   }
 }