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

js/js.translator/testData/box/incremental/coroutines.kt

@@ -21,7 +21,7 @@ fun builder(c: suspend () -> Unit) {
     c.startCoroutine(object : Continuation<Unit> {
         override val context = EmptyCoroutineContext
 
-        override fun resumeWith(result: SuccessOrFailure<Unit>) {}
+        override fun resumeWith(result: Result<Unit>) {}
     })
 }