Will Tebbutt

Actually, it might make sense just to replace `TINY` with a function called `add_tiny` or something to prevent accidentally forgetting to cast stuff.

#191 closed in favour of this PR. @mzgubic what's the status of this? Any chance we could push it through?

I _think_ the underlying problem is related to Example 2 from [this CRTU issue](https://github.com/JuliaDiff/ChainRulesTestUtils.jl/issues/213). Need to confirm by working through the things that are breaking though.

> I think the underlying problem is related to Example 2 from this CRTU issue. Need to confirm by working through the things that are breaking though. I did a...

This would make sense to me.

Could you provide an example @oxinabox ? I'm not really clear what sort of thing you're imagining.

And what we're proposing to do here would solve this problem, yes?

Oh, yeah, this particular example is. I think stuff would need to be nested in order to break it.

Ah, yes, this is generally problematic. I always do something similar to what you've done (I'll usually add the identity to `x * x'` to ensure it's reasonably well conditioned...

@oxinabox what's the status of this PR?