[RFC] subsystems vs. modules

[Attila Lendvai]

using :if-feature is not deterministic (it depends on e.g. the load
order of systems, unless fixed, see Robert's mail).

using subsystems that explicitly record dependencies is deterministic.

we once used asdf-system-connections, and the non-determinism that it
introduced was a source of long hours of headaches... once we moved on
to use subsystems and system names like :mylib+swank, it was all gone.
- attila