Allow dangling commas in argument lists

I was working on porting edge detection and kept adding dangling commas to argument lists, so let's just support that.

juno_frontend/src/lang.y

@@ -174,13 +174,12 @@ FuncDecl -> Result<Top, ()>
                           body : $11? }) }
   ;
 Arguments -> Result<Vec<(Option<Span>, VarBind)>, ()>
-  :                        { Ok(vec![]) }
-  | ArgBind                { Ok(vec![$1?]) }
-  | ArgumentsS ',' ArgBind { flatten($1, $3) }
+  : ArgumentsList { Ok($1?.into_iter().collect()) }
   ;
-ArgumentsS -> Result<Vec<(Option<Span>, VarBind)>, ()>
-  : ArgBind                { Ok(vec![$1?]) }
-  | ArgumentsS ',' ArgBind { flatten($1, $3) }
+ArgumentsList -> Result<VecDeque<(Option<Span>, VarBind)>, ()>
+  :                             { Ok(VecDeque::new()) }
+  | ArgBind                     { Ok(VecDeque::from([$1?])) }
+  | ArgBind ',' ArgumentsList   { let mut lst = $3?; lst.push_front($1?); Ok(lst) }
   ;
 ArgBind -> Result<(Option<Span>, VarBind), ()>
   : 'inout' VarBind { Ok((Some(span_of_tok($1)?), $2?)) }