Consider disallowing root nesting

[jplatte]

Before the 0.8.0 release, I would like to come back to nest("/", router) and nest_service("/", svc). I believe I suggested disallowing them (in favor of merge and fallback_service) before, but if so there must have been some pushback by David since we still allow both.

Any pointers or thoughts appreciated.

[jplatte]

The only thing I found is this: #2012
… which suggests I'm wrong about previous push-back and we should just disallow root nesting :)