The existing code makes two passes, one to compute the mean and another to compute the sum of squared differences from the mean.  A consequence of making two passes is that iterator inputs must be converted to a list before processing.  This throws away the memory saving advantages of iterators. 

The ostensible reason for the two pass code is that the single pass variant is numerically unstable when implemented with floating point accumulators.  However, this code uses fractions throughout, so the accumulation is exact.

Changing to a single pass saves memory, doubles the speed, and simplifies the upstream code in variance(), pvariance(), stdev(), and pstdev().