Fix inconsistent identifier spans in route codegen

[JOT85]

Previously, identifiers such as __req were output with inconsistent spans. For example, in query_decls, it was defined with Span::call_site(), but in request_guard_decl 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:

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

empty_get!(test);

In this case, the inconsistency arises because:

• In codegen_route, 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, 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.