Define the contents of ccobject_head_struct via CACHE_HEAD macro.
Don't rely on two different files to define a struct with the same
layout by convention.

Remove many levels of parentheses from the HOME and NON_GHOST_COUNT
macros.

Replace KEEP_THIS_ONE_AROUND_FOR_A_WHILE() macro with a call to the
accessed() function.  (Who comes up with these names :-).

Remove include of string.h in cPersistence.c.

Expand cPersistent_HEAD macro so that it has one line for each
attribute!

Use typedef to define PerCache as "struct ccobject_head_struct" and
use this for the entry cache slot in cPersistent_HEAD instead of
PyObject *.

In cPickleCache.c, reflow a bunch of long lines, add some whitespace,
and an XXX comment explaining that I don't understand a different
comment.