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/coroutines-experimental/jvm/src/kotlin/coroutines/experimental/migration/CoroutinesMigration.kt

@@ -79,7 +79,7 @@ private class ExperimentalContinuationMigration<T>(val continuation: Continuatio
 
 private class ContinuationMigration<T>(val continuation: ExperimentalContinuation<T>): Continuation<T> {
     override val context: CoroutineContext = continuation.context.toCoroutineContext()
-    override fun resumeWith(result: SuccessOrFailure<T>) {
+    override fun resumeWith(result: Result<T>) {
         result
             .onSuccess { continuation.resume(it) }
             .onFailure { continuation.resumeWithException(it) }