feat(tvix/eval): track pattern binding names

These need to be preserved at least for builtins.toXML.

Also, we incorrectly only wrote an <attrspat> in case ellipsis was true,
but that's not the case.

Change-Id: I6bff9c47c2922f878d5c43e48280cda9c9ddb692
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10686
Tested-by: BuildkiteCI
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: aspen <root@gws.fyi>

tvix/eval/src/value/function.rs

@@ -18,6 +18,10 @@ pub(crate) struct Formals {
 
     /// The span of the formals themselves, to use to emit errors
     pub(crate) span: Span,
+
+    /// Optionally tracks a name for all function arguments (args@ style).
+    /// Used by toXML.
+    pub(crate) name: Option<String>,
 }
 
 impl Formals {