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/bytecodeText/coroutines/doNotReassignContinuation.kt

@@ -4,7 +4,7 @@
 import kotlin.coroutines.*
 import kotlin.coroutines.intrinsics.*
 suspend fun suspendHere(): String = suspendCoroutineUninterceptedOrReturn { x ->
-    x.resumeWith(SuccessOrFailure.success("OK"))
+    x.resumeWith(Result.success("OK"))
 }
 
 suspend fun suspendThere(param: Int, param2: String, param3: Long): String {