Spans incorrect for map wrapper & backend err

[dvdsk]

Occurs on stable only.

Since Span.join() is still unstable we can not join multiple spans.

On stable:

• when wrapping map the call site span is used. That means that any errors relating to the key or value types (such as not implementing serde::Serialize) become hard to debug.
• when two back ends are specified (not allowed) only the second gets a span in the error.

This will be solved when proc_macro stablizes the join method.

[dvdsk]

Span::join tracking issue: rust-lang/rust#54725