Move some of the longer example code to external fragments, and

Move some of the longer example code to external fragments, and
include them using \verbatiminput.  This has the advantage that pages
can still break at reasonable places, and examples that go longer than
a page won't get cut off.

Make a few small markup adjustments for consistency.

Explain that PyObject_New() is not a C function but a polymorphic
beast that returns a pointer to the type that's passed as the first
arg.

Explain why type objects use the PyObject_VAR_HEAD.