Rename SuccessOrFailure to Result and hide Failure from ABI

* The members of Result are isSuccess, isFailure, exceptionOrNull, getOrNull
* The rest of API is implemented via inline-only extensions
* There are two internal functions to hide detailed mechanics of an internal
  Result.Failure class: createFailure and throwOnFailure
* Result.toString is explicit: either Success(v) or Failure(x)

See KT-26538

idea/testData/debugger/tinyApp/src/forTests/coroutines.kt

@@ -9,7 +9,7 @@ import java.util.concurrent.TimeUnit
 
 open class EmptyContinuation(override val context: CoroutineContext = EmptyCoroutineContext) : Continuation<Any?> {
     companion object : EmptyContinuation()
-    override fun resumeWith(result: SuccessOrFailure<Any?>) { result.getOrThrow() }
+    override fun resumeWith(result: Result<Any?>) { result.getOrThrow() }
 }