This will make a number of things easier in the future, as well as (finally!)
avoiding the Id-smashing problem which has plagued developers for so long.
Boy, I'm glad we're not using sup anymore. This update would have been
insane otherwise.
- add __BEGIN_DECLS and __END_DECLS,
- add a bunch of ``const'' qualifiers all over the place,
- rename the `private' struct member into `private_data' to avoid the
clash with the C++ keyword.
