Replace 'class object' with 'default object' in renderers and test data

Includes changes to decompiled text
Old syntax is used in builtins and project code for now

compiler/testData/codegen/boxWithStdlib/reflection/enclosing/lambdaInClassObject.kt

@@ -1,5 +1,5 @@
 class O {
-    class object {
+    default object {
         // Currently we consider <clinit> in class O as the enclosing method of this lambda,
         // so we write outer class = O and enclosing method = null
         val f = {}