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

libraries/stdlib/test/coroutines/CoroutinesReferenceValuesTest.kt

@@ -45,7 +45,7 @@ class CoroutinesReferenceValuesTest {
         bad.startCoroutine(object : Continuation<BadClass> {
             override val context: CoroutineContext = EmptyCoroutineContext
 
-            override fun resumeWith(result_: SuccessOrFailure<BadClass>) {
+            override fun resumeWith(result_: Result<BadClass>) {
                 assertTrue(result == null)
                 result = result_.getOrThrow()
             }