CLEANUP: debug: reorder definitions in bug.h

CLEANUP: debug: reorder definitions in bug.h

This file has become totally unreadable because it's littered with lots
of #if, and mixes unconditional and conditional declarations that have
been accumulating over time. Let's first reorganize it so that:

  - externs are defined first
  - isolated macros are defined second
  - enums/structs are defined third, optionally with their macros and
    inline functions if unconditional.
  - then the conditional stuff.

A few types were made permanent as there's no point hiding them behind
ifdefs, except complicating a developer's life in another file.

Now most common stuff is quickly found and the more involved stuff remains
at the end.