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/codegen/boxInline/suspend/callableReference/ordinaryParameter.kt

@@ -20,7 +20,7 @@ fun builder(c: suspend () -> Unit) {
         override val context: CoroutineContext
             get() = EmptyCoroutineContext
 
-        override fun resumeWith(result: SuccessOrFailure<Unit>) {
+        override fun resumeWith(result: Result<Unit>) {
             result.getOrThrow()
         }
     })

compiler/testData/codegen/boxInline/suspend/callableReference/suspendOfOrdinary.kt

@@ -18,7 +18,7 @@ fun builder(c: suspend () -> Unit) {
         override val context: CoroutineContext
             get() = EmptyCoroutineContext
 
-        override fun resumeWith(result: SuccessOrFailure<Unit>) {
+        override fun resumeWith(result: Result<Unit>) {
             result.getOrThrow()
         }
     })