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

compiler/testData/compileKotlinAgainstKotlin/coroutines/simple.kt

@@ -19,7 +19,7 @@ fun builder(x: suspend () -> Unit) {
     x.startCoroutine(object : Continuation<Any?> {
         override val context: CoroutineContext = EmptyCoroutineContext
 
-        override fun resumeWith(result: SuccessOrFailure<Any?>) {
+        override fun resumeWith(result: Result<Any?>) {
             result.getOrThrow()
         }
     })