Policy on alloc_prob == 0

[magnate]

At the moment there are a lot of items in object.txt with alloc prob 0, mostly there for V savefile compat. My suggestion is that they are removed altogether in v4, and we use alloc_prob of zero as a test for the INSTA_ART kinds created by add_kind(). This would then allow us to remove the INSTA_ART flag.

If there are other reasons for having objects with alloc_prob 0 (once #1565 is fixed, of course), this won't work. This ticket is a place for people to air any thoughts on the matter.

[magnate]

Just to note, I think #353 will require the dungeon spellbooks to stay in object.txt as kinds with alloc_prob 0 (because the actual spells in them are a property of the kind). We can still remove the INSTA_ART flag and use prob == 0 as the test for INSTA_ARTs, but we will still need at least these base items.