add comparisons between tangents and numbers

[oscardssmith]

useful for tests. I'm not sure whether this is a good idea, but it seems reasonable enough. @Keno

[mcabbott]

Xref https://github.com/JuliaDiff/ChainRulesCore.jl/issues/504

[mzgubic]

We removed some == methods because they were heavily invalidating https://github.com/JuliaDiff/ChainRulesCore.jl/issues/523

so it might be worth checking what the impact is here

[oscardssmith]

we should definitely check in validations, but I think defining == for concrete thunk subtypes should be much better on that front.