Unify decision tactics

We currently have 3 tactics case_decide, case_bool_decide, case_option_guard that are very similar but work on decide, bool_decide, and guard, respectively. Tactics are pretty hard to discover (there is no useful SearchAbout for them and we don't document a "tactic index" for std++), so this is bad -- I didn't even know case_option_guard was a thing until last week.

I think we should unify these 3 tactics into one, so that one does not have to independently discover all 3 of them.