Replace Router::{map_inner, tap_inner_mut} by macros

[jplatte]

… so that #[track_caller] works.

Adding #[track_caller] to the two functions would not have the same effect as the closures passed to both would also need the attribute, but it's currently not supported on closures:
rust-lang/rust#87417

Motivation

We use #[track_caller] in a few places where we panic on invalid user input, to have the panic message point at the relevant method call in the user's code. A few of those, including the one for the new routing syntax, don't work as intended; the panic message still points at something inside axum.

Solution

Replace the internal helpers that break #[track_caller] by macros.