D Mir: standard deviation speed

[jmh530]

On Wednesday, 15 July 2020 at 11:26:19 UTC, 9il wrote:
> [snip]
>>
>> @fmamath private double sd(T)(Slice!(T*, 1) flatMatrix)
>
> @fastmath violates all summation algorithms except `"fast"`.
> The same bug is in the original author's post.

I hadn't realized that @fmamath was the problem, rather than 
@fastmath overall. @fmamathis used on many mir.math.stat 
functions, though admittedly not in the accumulators.