Helper function for declaring rule lists.

[bddap]

Context: https://github.com/docknetwork/rify/pull/10/files/2eec95882bf018c3678f1f168be2fd3192bb76f3#r590589241

The function decl_rules is declared but only exposed to unit tests. Users could benefit from a public version.

The public version shouldn't panic. Instead it should return a Result.