Rewrote the basic section of the chapter on defining new types.

Changed the example to show how to create types the new way:

- Use a class new method rather than a new function.

- Use self->ob_type->tp_free in deallocators

- Use attribute descriptors rather than set/getattr methods.

- Make the type usable as a base type.

I split the example into 3 parts:

1. The minimal new type

2. Adding attributes and methods.

3. Finer control over attributes.

It's much simpler to define builtin types. These updates hopefully
show this.

I also made minor wording changes in two other places.

I still need to update xxobject.c