D Mir: standard deviation speed

[9il]

On Wednesday, 15 July 2020 at 02:08:48 UTC, 9il wrote:
> On Tuesday, 14 July 2020 at 19:04:45 UTC, tastyminerals wrote:
>> @fastmath private double sd0(T)(Slice!(T*, 1) flatMatrix)
>
> @fastmath shouldn't be really used with summation algorithms 
> except the `"fast"` version of them. Otherwise, they may or may 
> not behave like "fast".

`mean` is the summation algorithm too