Remove `SOD_CAR' from the public interface.

Remove `SOD_CAR' from the public interface.

It's now called `SOD__CAR', no longer documented, and in a new, clearly
marked `preliminaries utilities' section of the header file.  This new
section precedes the header-file inclusions because they can in
principle depend on the definitions made here.

This macro probably won't change, but I reserve the right to make the
internal utilities be weird in ways which won't break the way Sod uses
them, but might break other things which rely on them.