Want SDT probes for `gen_desc()`

It would be nice to have an SDT probe fire anytime a new action descriptor is generated (gen_desc()). This probe could either be generic to all actions or specific to each action.

1. Generic: Have each action descriptor implement Display and have a generic action-desc-created even whose single argument is a string.

2. Specific: Have each action create its own SDT probe, e.g. something like snat-desc-created. Then create a repr(C) struct holding the important information of such a descriptor. The probe would then pass a pointer to said structure, allowing the consumer to inspect and format the information in any manner they desire.

Option 2 obviously gives more control, but it also means more code, and making sure that new actions add a probe. Perhaps we can enforce probe creation somehow via a trait? For example:

trait ActionDesc {
    fn fire_create_probe(&self);
}

I lean towards option 2, if only for the fact that it can be extremely useful, when debugging, to be able to programmatically filter matches when using DTrace. Option 1 would require hacks like strstr() or post-processing with awk.