This restores the Causer and ungrouper interfaces and support for them,
because someone might have code using them already.

Also, this makes errs.Is() work (almost) the same when building with go
< 1.20 as it does when building with go 1.20. To be specific, this adds
support use of the `err.Is(target)` idiom.

The MyClass.Has(err) idiom has never been anyone's favorite thing. It
was a necessary compromise given what was available to us at the time.
But with the help of errors.Is(), maybe something like this is better?

    MyClass := errs.Class("foo")
    err := MyClass.Wrap(io.EOF)
    if errors.Is(err, MyClass.Instance()) {
        fmt.Println("err is an instance of MyClass")
    }

It's still a little clunky, since we have to have the .Instance() method
there at all. I think it's necessary to avoid having Class implement the
error interface itself, and I think that's always been an anti-goal.

Suggestions welcome for a better name.