This patch implements some new aggregate functions defined by SQL2003:
stddev_pop(), stddev_samp(), var_pop(), and var_samp(). stddev_samp()
and var_samp() are identical to the existing stddev() and variance()
aggregates, so I've made the latter aliases for the former.
I noticed that SQL2003 does not allow DISTINCT to be specified for these
aggregate functions. I can't really see the rationale for this
restriction, and it would be fairly ugly to implement as far as I can
tell. Thoughts?
-Neil
