Undo unnecessary changes to stdlib

Partially revert "[stdlib] Reduce usage of extension functions"

libraries/stdlib/src/kotlin/coroutines/CoroutineContextImpl.kt

@@ -98,7 +98,7 @@ internal class CombinedContext(
 
     override fun toString(): String =
         "[" + fold("") { acc, element ->
-            if (acc.length == 0) element.toString() else "$acc, $element"
+            if (acc.isEmpty()) element.toString() else "$acc, $element"
         } + "]"
 
     private fun writeReplace(): Any {
@@ -116,11 +116,6 @@ internal class CombinedContext(
             private const val serialVersionUID: Long = 0L
         }
 
-        private fun readResolve(): Any {
-            var result: CoroutineContext = EmptyCoroutineContext
-            for (element in elements)
-                result = result + element
-            return result
-        }
+        private fun readResolve(): Any = elements.fold(EmptyCoroutineContext, CoroutineContext::plus)
     }
 }

libraries/stdlib/src/kotlin/util/Standard.kt

@@ -135,7 +135,7 @@ public inline fun <T> T.takeUnless(predicate: (T) -> Boolean): T? {
 public inline fun repeat(times: Int, action: (Int) -> Unit) {
     contract { callsInPlace(action) }
 
-    for (index in 0 .. (times - 1)) {
+    for (index in 0 until times) {
         action(index)
     }
 }