List slays tweaks

With this I'm trying to get a way to list the actual slays on an object by index as well as by description, so that the raw data form of what-the-player-knows-about-an-object can be exposed (e.g. to borgs, more exotic UIs) as well as described in text.

The alternative approach is to add another array to the current list_slays() functions that can be filled with a slay index list, but this seemed a bit nicer as it makes it more straightforward to split the gather-info-about-an-object from the display-info-about-an-object later if we decide that's worth the effort. I can redo it in the other form though if that's more acceptable for now.

There's also a couple of other tweaks that probably should've gone in a separate request, but never mind eh?