Should we generate AbstractArray rand_tangent for all AbstractArray subtypes?

See discussion at https://discourse.julialang.org/t/chainrulescore-rrule-for-custom-struct-does-the-pullback-need-to-support-composite-explicitly/56734

We can maybe do something with zero and similar like Zygote does for getindex
https://github.com/FluxML/Zygote.jl/blob/956cbcf3c572c0eb09c146189bb38b1b434634ff/src/lib/array.jl#L47-L49
I find thought when i was working on the getindex for ChainRules (https://github.com/JuliaDiff/ChainRules.jl/pull/240) trying to work this out for general case is actually really hard, i gave up and restricteds to x::Array{<:Number}.

But maybe for rand_tangent we don't need to do so well, we just need to make something that is more likely to be what the user expects than Composite is.
It can still screw up, but as long as it screws up less, or with a clearer error.

Start with the linked Discourse discussion and ChainRules getindex PR #240 to understand the unresolved behavior. Compare the referenced Zygote implementation in src/lib/array.jl with the current rand_tangent behavior. Done means the project has an agreed approach for AbstractArray subtypes, with its failure behavior and expected coverage defined.