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/classes/kt508.kt

@@ -2,7 +2,7 @@ package mult_constructors_3_bug
 
 public open class Identifier() {
     private var myNullable : Boolean = true
-    class object {
+    default object {
         open public fun init(isNullable : Boolean) : Identifier {
             val __ = Identifier()
             __.myNullable = isNullable