enable interfaces for equals and toString

Generic Interfaces could be helpful, that allow alternative `toString()` behavior, e.g.

```java
interface Printable {
  A getA();
  B getB();
  @ToString
  default String display() {
   return getA() + ":" + getB();
  }
}

assertThat(xray(obj).to(Printable.class).toString(), equalTo("<A.toString()>:<B.toString()>"))
```

Similar for `equals(Object)` behavior.
