Middlewares: execution order + docs

[LukeMathWalker]

While documenting the execution order of middlewares, I realised that the existing implementation resulted in some unintuitive behaviour (i.e. a middleware would apply to all routes in the same Blueprint, even if the middleware was registered after a route).
This PR changes the behaviour to align with the user's intuition.

Internals

Refactorings lead to other refactorings: I had to change the internal representation of a Blueprint to fix the issue with middlewares. I took the opportunity to remove the internals module from pavex entirely. It is now in a separate crate, pavex_bp_schema, which is not exposed through pavex's public API. It is an (implicit) dependency of Blueprint::persist and Blueprint::load.