Add IterableOnce operations to IterableOnceOps

Fixes scala/bug#11675

This change is backward compatible, because nothing is removed.
It should also be forward compatible, because `IterableOnceOps[A]` had
`IterableOnce[A]` as a self-type on 2.13.0, meaning that code relying on the
newly introduced methods also worked on 2.13.0 because any `IterableOnceOps[A]`
instance was also effectively an `IterableOnce[A]`.

Author: julienrf

build.sbt

@@ -94,7 +94,13 @@ val mimaFilterSettings = Seq(
     ProblemFilters.exclude[MissingClassProblem]("scala.reflect.runtime.JavaMirrors$JavaMirror$typeTagCache$"),
     ProblemFilters.exclude[DirectMissingMethodProblem]("scala.reflect.api.TypeTags.TypeTagImpl"),
     ProblemFilters.exclude[DirectMissingMethodProblem]("scala.reflect.api.Universe.TypeTagImpl"),
-  ),
+    ProblemFilters.exclude[ReversedMissingMethodProblem]("scala.collection.IterableOnceOps.stepper"),
+    ProblemFilters.exclude[ReversedMissingMethodProblem]("scala.collection.IterableOnceOps.iterator"),
+    ProblemFilters.exclude[ReversedMissingMethodProblem]("scala.collection.IterableOnceOps.knownSize"),
+    ProblemFilters.exclude[DirectMissingMethodProblem]("scala.collection.IterableOnceOps.stepper"),
+    ProblemFilters.exclude[DirectMissingMethodProblem]("scala.collection.IterableOnceOps.iterator"),
+    ProblemFilters.exclude[DirectMissingMethodProblem]("scala.collection.IterableOnceOps.knownSize")
+  )
 )
 
 // Save MiMa logs

src/library/scala/collection/IterableOnce.scala

@@ -296,7 +296,8 @@ object IterableOnce {
   * @define coll collection
   *
   */
-trait IterableOnceOps[+A, +CC[_], +C] extends Any { this: IterableOnce[A] =>
+trait IterableOnceOps[+A, +CC[_], +C] extends Any with IterableOnce[A] {
+
   /////////////////////////////////////////////////////////////// Abstract methods that must be implemented
 
   /** Produces a $coll containing cumulative results of applying the

test/files/neg/abstract-report.check

@@ -1,6 +1,6 @@
 abstract-report.scala:1: error: class Unimplemented needs to be abstract.
 Missing implementations for 6 members. Stub implementations follow:
-  // Members declared in scala.collection.IterableOnce
+  // Members declared in scala.collection.IterableOnceOps
   def iterator: Iterator[String] = ???
 
   // Members declared in scala.collection.IterableOps

test/files/neg/abstract-report2.check

@@ -54,7 +54,7 @@ class Baz[T] extends Collection[T]
       ^
 abstract-report2.scala:15: error: class Dingus needs to be abstract.
 Missing implementations for 7 members. Stub implementations follow:
-  // Members declared in scala.collection.IterableOnce
+  // Members declared in scala.collection.IterableOnceOps
   def iterator: Iterator[(Set[Int], String)] = ???
 
   // Members declared in scala.collection.IterableOps