Annotate panicking functions with #[track_caller]

[davidpdrsn]

This annotates panicking functions with #[track_caller] so the error message points to where the user added the invalid router, rather than somewhere internally in axum.

Before:

thread 'routing::tests::foobar' panicked at 'Invalid route "/:b": insertion failed due to conflict with previously registered route: /:a', axum/src/routing/mod.rs:174:13

After

thread 'routing::tests::foobar' panicked at 'Invalid route "/:b": insertion failed due to conflict with previously registered route: /:a', axum/src/routing/tests/mod.rs:667:10

This should make it even easier to hunt down which route actually causes the conflict.

[jplatte]

Your "Before" and "After" are equal 😆

[davidpdrsn]

Whoops! Fixed 😅