Support optional diff formatter fn for eq

[orls]

I'm keen to see progress on more useful output when expectations fail – e.g. #130 and various historical tickets on the original repo.

PR #154 proposes directly using go-cmp's cmp.Diff internally for this purpose. That would be great from my POV. But maintainers have given feedback about change scope and extra dependency.

So this is an alternative approach, which tries to assuage those concerns... or at least revive the conversation.

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This implements a new Controller option,WithDiffFormatter. It takes a function that is called with the expected and actual values, only when the Eq matcher fails. I think this would cover the vast majority of use-cases.

This gives users freedom to do what they like, including passing to cmp.Diff or a similar lib.

I think this is a significant improvement over the status quo:

• because true diff-style output, for deep/complex objects, isn't possible with the current GotFormatter and WantFormatter.
• because it avoids the need for every expectation to be set up with custom formatting wrappers.

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