Allow ASC/DESC direction modifiers for index columns.



Almost every database has this, and it is a pest to have to write a
function for every index column you want descending.



Now, it is not technically difficult (nothing more than -compare(x,y)
instead of compare(x,y) to produce the ordering) but it would be much
better to simply add it to the language.



It would obviously require at least two changes, one to the grammar and
one to the indexing code.