Unneeded exports cause confusion about the module interfaces.
Make all functions static. Add a short header comment.
The forward decls are just code clutter; remove them, by moving
their references after definitions. This is another file which
never needed even one internal forward declaration.
Remove unneeded indirection for the write_memory() method. Make
a table static, remove a can't-happen case with nasty exit().
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>