Previously, identifiers such as `__req` were output with inconsistent
spans. For example, in
[`query_decls`](https://github.com/rwf2/Rocket/blob/28891e8072136f4641a33fb8c3f2aafce9d88d5b/core/codegen/src/attribute/route/mod.rs#L43),
it was defined with `Span::call_site()`, but in
[`request_guard_decl`](https://github.com/rwf2/Rocket/blob/28891e8072136f4641a33fb8c3f2aafce9d88d5b/core/codegen/src/attribute/route/mod.rs#L127)
as `guard.ty.span()`. The problem is, when used with macros, these spans
can be different and cause the identifiers to fail to resolve.

A simple failure example is:

```rust
macro_rules! empty_get {
    ($name:ident) => {
        #[get("/")]
        async fn $name() {}
    }
}

empty_get!(test);
```

In this case, the inconsistency arises because:

- In
  [`codegen_route`](https://github.com/rwf2/Rocket/blob/28891e8072136f4641a33fb8c3f2aafce9d88d5b/core/codegen/src/attribute/route/mod.rs#L380),
  no span for `__req` is specified, which I believe means it defaults to
  `Span::call_site()`, so inside `macro_rules!`, then
- in
  [`responder_outcome_expr`](https://github.com/rwf2/Rocket/blob/28891e8072136f4641a33fb8c3f2aafce9d88d5b/core/codegen/src/attribute/route/mod.rs#L299),
  the span used is from the identifier of the function, which is outside
  of `macro_rules!`.

This causes the `__req` in `responder_outcome` not to resolve to the
`__req` defined in the `monomorphized_function` arguments.

The span is now always `Span::call_site()`, which resolves these issues.

I believe `call_site` is acceptable, since it was already used as the
span for these variables elsewhere. In addition, I didn't spot anywhere
in the code where this could cause a problem.