Make concurrency-friendly by adding a type that captures desired config options. Existing API fully preserved.

[acorello]

Introduced Differ, a struct capturing all the config options (including the one passed as "flag"). It now hosts the implementation of deep.Equal as the method func(Differ)Compare.

Original deep.Equal preserved. Internally it delegates to a new Differ, which is configured from the package-level settings.

Original tests have zero changes ensuring things work as expected.

I'm not confident the naming chosen is consistent / clear. Please help.

I've used Compare instead of Equal as method name on Differ because the Equal is conventionally reserved for fun (Something) Equal(any) bool signature.

I've named the config struct Differ to convey the "thing that will calculate a diff" (even though it's just a wrapper for the config). Perhaps Comparer/Comparison would sound better?

The original Equal returned a []string. I'm returning a named type Delta instead, to encapsulate implementation details and facilitate use/addition of common predicates (eg delta.IsEmpty(), (Delta) Has(diff string) bool, etc.)…
But I read the Type Hiding anti-pattern on your blog. Would you (or the library users) prefer a plain []string instead?