Allow GROUPING to have multiple arguments, like GROUPING_ID

When initially added to the SQL standard GROUPING took one argument, a column reference. Now it can have multiple arguments, like the GROUPING_ID function.

In SQL standard 2014, see section 6.9 <set function specification>, and feature T433, "Multiargument GROUPING function".

This change would extend GROUPING to allow one or more arguments (i.e. what GROUPING_ID currently does), and make GROUPING_ID a synonym for GROUPING. GROUP_ID is not affected, and continues to take zero arguments.