add syntax for new grouping expressions

TODO

@@ -7,6 +7,19 @@ group by extensions. Question: some of the syntax can be represented
    by app and row ctor, should this be reused or new syntax created
    (the standard has special grammar for cube and rollup).
 
+SELECT EmpId, Yr, SUM(Sales) AS Sales
+FROM Sales
+GROUP BY GROUPING SETS((EmpId, Yr), (EmpId), ())
+
+SELECT EmpId, Yr, SUM(Sales) AS Sales
+FROM Sales
+GROUP BY GROUPING SETS((EmpId, Yr), (EmpId))
+
+{ (grouping_column[, ...]) | ROLLUP (grouping_column[, ...]) |
+CUBE (grouping_column[, ...]) | GROUPING SETS (grouping_set_list) |
+( ) | grouping_set, grouping_set_list }
+
+
 collate? -> postfix operator which binds very tightly:
 a < 'foo' collate 'C'
 ->
@@ -15,6 +28,7 @@ Op "<" [Iden "a", SpecialOp "collate" [StringLit 'foo', StringLit
   also postfix in order by:
 select a from t order by a collate 'C': add to order by syntax, one
    collation per column
+have to do fixity for this to work
 
 much more table reference tests, for joins and aliases etc.