Disallow nesting at the root of the router

[jplatte]

Motivation

It's confusing, there isn't really any nesting going on.

Solution

Remove support for nesting at the root. merge can fully replace nest("" | "/", ...), fallback_service can fully replace nest_service("" | "/", ...).

Closes #2651.

[yanns]

I don't use this feature, so I cannot judge if the change is good for the users.
From the code point of view, all is looking good.

[jplatte]

I'm pretty confident that the suggested replacements are better suited for all use cases.

If not, it's backwards-compatible to revert this.