feat: add class name storage and expose via name method on class objects

Store the class name string in `ObjClass` during compilation and class creation, and expose it through a new `class_name` native method. Also update `object_toString` to return the class name for class instances and "instance of <name>" for regular instances.

test/class/name.wren

@@ -0,0 +1,9 @@
+class Foo {}
+
+IO.print(Foo.name) // expect: Foo
+IO.print(Foo.type.name) // expect: Foo metaclass
+
+// Make sure the built-in classes have proper names too.
+IO.print(Object.name) // expect: Object
+IO.print(Class.name) // expect: Class
+IO.print(Bool.name) // expect: Bool