gnat-8 migration

[Nicolas Boulenguez <nicolas@debian.org>]

Hello.

All Ada packages seem ready for the migration to GCC-8, and the
release team has given their consent [1].

Most packages only require a trivial source upload to unstable with a
changelog entry stating:
  * Rebuild in unstable for gnat-8 transition.
while a few exceptions only require a rebuild inside unstable.

Unless a co-maintainer objects, I would like to proceed with the
packages I co-maintain around the 28th of october.
Phil, are you OK with this date?

[1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=908931