Make macros very careful about what they expect to be in scope

[oxinabox]

Follow up to #318 about doing it the right way. (rather than just enough to fix the symptom)

From #317 @devmotion said:

but IMO it is the wrong fix. Instead one should apply esc only locally, wherever needed: currently definitions such as @non_differentiable Base.length(kwargs::AbstractVector) or @non_differentiable Base.length(rrule::AbstractVector) will lead to multiple arguments of the same name in the rrule and frule definitions.

@simeonschaub said in #319 (comment)_

This will still fail if one only does using ChainRulesCore: @scalar_rule. I think it would be better to just interpolate rrule directly into the AST.

We should do this.
and we should text it via addign to the isolated test modules just using ChainRulesCore: @scalar_rule, @non_differentiable
which brings as little into scope as possible