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-engine/src/scope.rs

@@ -548,8 +548,7 @@ impl<'e, 's> ScopeData<'e, 's> {
                             Value::String { val: key, span },
                             Value::String {
                                 val: String::from_utf8_lossy(
-                                    self.engine_state
-                                        .get_span_contents(&alias.wrapped_call.span),
+                                    self.engine_state.get_span_contents(alias.wrapped_call.span),
                                 )
                                 .to_string(),
                                 span,