Don't allow extracting MatchedPath in middleware for nested routes

[davidpdrsn]

This changes MatchedPath so it cannot be extracted in middleware of nested routes. This means users wont see the internal wildcard route (/nested/*__private__axum_nest_tail_param) that actually matched. This is an implementation detail and shouldn't be exposed.

I didn't make MatchedPath accessible in response extensions like discussed in #1441 because thats not a breaking change and shouldn't block shipping 0.6.0

I also reworked the tests so there are multiple smaller tests instead of one big one. Should be easier to read.

Part of #1441

[dcormier]

It would be nice if the error returned by impl FromRequestParts for MatchedPath made it clear that it failed because there's a rested router. It would give people some quick guidance that it will work if they refactor their router stack, if they can (this is what I did based on the conversation in #1441).

[davidpdrsn]

It would be nice if the error returned by impl FromRequestParts for MatchedPath made it clear that it failed because there's a rested router. It would give people some quick guidance that it will work if they refactor their router stack, if they can (this is what I did based on the conversation in #1441).

That would mean adding support for MatchedPath in nested routers would be a breaking change. I'm also not sure how users would actually handle the error differently from the general MatchedPathRejection. Personally I think documenting it is sufficient.

[dcormier]

Yes, I agree. I don't think they would be able to handle it differently. I was thinking purely for discoverability of the fact that it won't work with nested routers.

[jplatte]

Outside of some small code style things, looks good!