Allow Routers to inherit state, take 2

[jplatte]

Motivation

Closes #1326.

Solution

Make Router<S, _> no longer a type that necessarily holds a state instance of type S. Instead, we track (at runtime, to avoid making the generics too complex) whether a given Router instance wants to inherit its state.

Status

I implemented router merging which was much easier than splitting nest into nest and nest_service. The fallback handler (currently stubbed out) will also need some special handling, but I don't think it should be a showstopper.

[jplatte]

@davidpdrsn Have you had a look? I might have some time today or in the coming days to come back to this.

[davidpdrsn]

I like the direction of this! I think its a good balance between usability and type safety.

[davidpdrsn]

I implemented router merging which was much easier than splitting nest into nest and nest_service.

I'm fine with merging this without support for nest. Then we can do that in a follow up PR.

[jplatte]

So I basically re-did everything except boxed.rs with the latest force-push. Things should now be much cleaner.

[jplatte]

I also think we can get rid of passing state via Extension with the boxed handler approach, but that is a big refactoring on its own that I don't want to mix into this PR.

[jplatte]

Rebased to fix the unrelated trybuild failure.

[jplatte]

Added nest, it was sooo easy. Hopefully the implementation is correct 😄

[davidpdrsn]

I think it looks good! I'll write some docs in a follow-up PR.

Wanna update the changelog? Then we can merge this 🚀

EDIT: We already have #1313 so wont file a separate issue.