Inconsistencies between `Relation.Binary.Definitions` and `Relation.Unary` and `Relation.Nullary`

[jamesmckinna]

Maybe it's worth considering reconciling all the relational hierarchy at each arity wrt definitions of:

• Irrelevant
• Recomputable
• Stable
• WeaklyDecidable
• Decidable
• Satisfiable
• Empty
• Universal
• etc. (the list may grow...)

because otherwise, we have a lot of redundant repetition on the one hand, but with 'gaps' on the other. There's some overlap/degeneracy at arity 0 for eg Satisfiable/Universal (the identity) and Empty (negation), but I think that is harmless...

Maybe there are good reasons to do with dependencies (eg. Relation.Nullary.Decidable.Core.recompute can't be typed with a Nullary definition of Recomputable... unless we bite the bullet and lift it out into a Relation.Nullary.Definitions etc. But maybe that's not such a bad idea, either? See also #2243 for my current approach to this particular predicate...

UPDATED: partial fix in #2259 (and largely orthogonal to #2243 )

[jamesmckinna]

Reconsidering whether to keep this open, I think on balance after the merge of #2259 that this one can be considered closed by that PR... the other cases being a little too non-uniform to be worth pursuing?