Panic instead of silently discarding fallbacks

[davidpdrsn]

This introduces two new possible panics when constructing routers:

• If merging two routers that each have a fallback. Previously that left
  side fallback would be silently discarded.
• If nesting a router that has a fallback. Previously it would be
  silently discarded.

Overall this should make things more explicit and users shouldn't have
to worry "why isn't my fallback" working.

Fixes #488

[jplatte]

If nesting a router that has a fallback.

Why is this? It should be possible to have a fallback for /foo/unrecognized that's different from /unrecognized, no?

[davidpdrsn]

Its unfortunately no so straight forward with how nest is currently implemented:

If the service you're trying to nest is actually a Router then we simply clone all the routes from the nested router to the outer router. That means we end up one "flat" router in the end which removes pretty much all the complexity nesting otherwise leads to.

Adding a wildcard route like .route("/nest-path/*axum_fallback", nest_fallback) doesn't work since that conflicts with the nested routes.

[jplatte]

Adding a wildcard route like .route("/nest-path/*axum_fallback", nest_fallback) doesn't work since that conflicts with the nested routes.

Oh, so while /foo and /:var don't conflict, /foo and /*wildcard do?

[davidpdrsn]

Yep. That was the source of the whole nest("/", ServeDir::new(...)) thing.