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-cmd-lang/src/core_commands/use_.rs

@@ -71,7 +71,7 @@ This command is a parser keyword. For details, check:
 
                 // See if the module is a file
                 let module_arg_str = String::from_utf8_lossy(
-                    engine_state.get_span_contents(&import_pattern.head.span),
+                    engine_state.get_span_contents(import_pattern.head.span),
                 );
 
                 let maybe_file_path = find_in_dirs_env(