Reduce scope of quote_spanned on SerializeWith wrapper

[dtolnay]

This leaves more of the generated code accurately identified as generated, avoiding the dead_code and needless_lifetimes warnings that originated from #2558.

[Mingun]

@dtolnay, for better understanding, could you explain, how this change fixes the warnings? rustc considers quote! code blocks as generated and quote_spanned! as not generated?

[dtolnay]

Tokens with a call_site span are understood as being macro-generated. It is not a difference between quote vs quote_spanned. Quote produces call_site spanned tokens, but if you passed a span to quote_spanned that happened to be call_site, then those tokens also would be considered generated despite coming from a quote_spanned expansion.

More precisely, the span's "resolution" being call_site is the thing that matters, as opposed to the span's "location" being call_site. So user_span.resolved_at(Span::call_site()) is generated while user_span.located_at(Span::call_site()) is not generated.