Replace &Span with Span since Span is Copy (#9770)

# Description
`Span` is `Copy`, so we probably should not be passing references of
`Span` around. This PR replaces all instances of `&Span` with `Span`,
copying spans where necessary.

# User-Facing Changes
This alters some public functions to take `Span` instead of `&Span` as
input. Namely, `EngineState::get_span_contents`,
`nu_protocol::extract_value`, a bunch of the math commands, and
`Gstat::gstat`.

crates/nu_plugin_gstat/src/nu/mod.rs

@@ -22,6 +22,6 @@ impl Plugin for GStat {
 
         let repo_path: Option<Spanned<String>> = call.opt(0)?;
         // eprintln!("input value: {:#?}", &input);
-        self.gstat(input, repo_path, &call.head)
+        self.gstat(input, repo_path, call.head)
     }
 }