Improve the standard deviation computation

This does just two passes, instead of three, over the list. This reduces
the overall runtime well enough (~25%) in some tests, but it's not
reproducible using profiling, so I don't know how much the function
itself is being sped-up.

Note: this is written via `seq`s, and not BangPatterns. Since it's just
one case, adding BangPatterns just for it wasn't a big gain.

Thanks to Lécz Balázs for the impetus to improve this!

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Balazs Lecz <leczb@google.com>