Expose values through ValueWithSpan

[xitep]

• This exposes values in the AST as ValueWithSpan allowing clients to get hold of the location/span for 1) error reporting and 2) associating metadata with the visited values (the start location can be used to make up an identifier)
• Tests work only because the parser in the tests sets all tokens' spans to "empty"; ie. there is not "magic" wrt to PartialEq/Eq like for the AttachedToken
• I'm not entirely sure it's the right approach, but it's at least consistent with other items (e.g. Ident) exposing their span in the AST
• Relevant to [EPIC] Complete Span (source location) information / feature #1548
• The Deref<Target = Value> for ValueWithSpan would make an upgrade to this version less hassle:

    fn pre_visit_value(&mut self, value: &ValueWithSpan) -> ControlFlow<Self::Break> {
        match **value { // <--- access `&value.value` through deref via `**`
            Value::Number(_, _) => eprintln!("number"),
            _ => eprintln!(".."),
        }
 ...