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/tests-common/tests/org/jetbrains/kotlin/coroutineTestUtil.kt

@@ -17,7 +17,7 @@ fun createTextForHelpers(isReleaseCoroutines: Boolean): String {
     val emptyContinuationBody =
         if (isReleaseCoroutines)
             """
-                |override fun resumeWith(result: SuccessOrFailure<Any?>) {
+                |override fun resumeWith(result: Result<Any?>) {
                 |   result.getOrThrow()
                 |}
             """.trimMargin()
@@ -30,7 +30,7 @@ fun createTextForHelpers(isReleaseCoroutines: Boolean): String {
     val handleResultContinuationBody =
         if (isReleaseCoroutines)
             """
-                |override fun resumeWith(result: SuccessOrFailure<T>) {
+                |override fun resumeWith(result: Result<T>) {
                 |   x(result.getOrThrow())
                 |}
             """.trimMargin()
@@ -46,7 +46,7 @@ fun createTextForHelpers(isReleaseCoroutines: Boolean): String {
     val handleExceptionContinuationBody =
         if (isReleaseCoroutines)
             """
-                |override fun resumeWith(result: SuccessOrFailure<Any?>) {
+                |override fun resumeWith(result: Result<Any?>) {
                 |   result.exceptionOrNull()?.let(x)
                 |}
             """.trimMargin()
@@ -62,7 +62,7 @@ fun createTextForHelpers(isReleaseCoroutines: Boolean): String {
     val continuationAdapterBody =
         if (isReleaseCoroutines)
             """
-                |override fun resumeWith(result: SuccessOrFailure<T>) {
+                |override fun resumeWith(result: Result<T>) {
                 |   if (result.isSuccess) {
                 |       resume(result.getOrThrow())
                 |   } else {