`srw` should rewrite everywhere in goal by default, not just first occurrence

[dranov]

Maybe also print a helper message of how to rewrite a single occurrence if multiple are rewritten?

[volodeyka]

I strongly suspect this is the case only for unfolding. I don't know whether we should do something specific about that. Likewise, I remember specifying the exact location of unfold in ssreflect.coq quite frequently. Maybe let's use ?name for now?