Rework class objects

Class objects have name "Default" by default
Do not produce synthetic class objects
Class objects have new semantics:
    class "A" has class object "B" if literal "A" can be used as a value of type "B"
Class objects act like ordinary nested objects
    i.e. are accessible from class scope via getClassifier()
Jvm backend: class object fields and class have the name of class object ("Default")
	as opposed to special "OBJECT$" and "object"
Serialization: only the name of class object is needed to serialize data

compiler/builtins-serializer/src/BuiltInsSerializer.kt

@@ -201,11 +201,6 @@ public class BuiltInsSerializer(private val dependOnOldBuiltIns: Boolean) {
         writeClass(classDescriptor, classProto)
 
         serializeClasses(classDescriptor.getUnsubstitutedInnerClassesScope().getDescriptors(), serializer, writeClass)
-
-        val classObjectDescriptor = classDescriptor.getClassObjectDescriptor()
-        if (classObjectDescriptor != null) {
-            serializeClass(classObjectDescriptor, serializer, writeClass)
-        }
     }
 
     private fun serializeClasses(