Use consistent names for `Decidable` combinators (#2842 redux)

[jamesmckinna]

As #2843 makes clear, this issue proliferates beyond the Relation.Nullary.Decidable.Core combinators, but how to handle that (downstream!) should probably be separate issues (incl. eg. #2845 ), but with callbacks to #2842 ...

Shopping list:

• [ fix ] Relation.Nullary.Decidable.Core names for combinators #2843 (comment)
• [ fix ] Relation.Nullary.Decidable.Core names for combinators #2843 (comment)
• [ fix ] Relation.Nullary.Decidable.Core names for combinators #2843 (comment)
• [ fix ] Relation.Nullary.Decidable.Core names for combinators #2843 (comment)
• [ fix ] Relation.Nullary.Decidable.Core names for combinators #2843 (comment)