add row_sequence UDF

This is a poor man's answer to the standard analytic function row_number(); it assigns a sequence of numbers to rows, starting from 1.

I'm calling it row_sequence() to distinguish it from the real analytic function, so that once we add support for those, there won't be any conflict with the existing UDF.

The problem with this UDF approach is that there are no guarantees about ordering in SQL processing internals, so use with caution.