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/box/functions/kt2716.kt

@@ -1,7 +1,7 @@
 package someTest
 
 public class Some private(val v: String) {
-    class object {
+    default object {
         public fun init(v: String): Some {
             return Some(v)
         }