take address of field for comparison

I'm using [intsets.Sparse](https://pkg.go.dev/golang.org/x/tools/container/intsets#Sparse) as a field in a struct:

```go
type T struct {
v intsets.Sparse
// other fields
}
```

When traversing `T`, the field `v` is copied, causing [all methods to fail](https://cs.opensource.google/go/x/tools/+/refs/tags/v0.2.0:container/intsets/sparse.go;l=264). This precludes the use of `Transformer`.

Ideally, there would be an `Option` that would take the address of the field for use with other `Option`s. Notably, `Sparse` has an `Equals(*Sparse) bool` method, so perhaps an alternate idea is to allow different equality methods? But I'm not sure whether `go-cmp` will take the address of a field in order to invoke the `Equal` method.