Added and used Numerics.Vector{128|256}Count extension

[gfoidl]

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Description

Instead of typing

nuint n = (uint)dest.Length / (uint)Vector256<float>.Count;

everytime, there's now a helper that shortens this to

nuint n = dest.Vector256Count<float>();

Cf. #2419 (comment)

[gfoidl]

COMPlus prefix changed with .NET 6 to be DOTNET.
The COMPlus is still supported, but may get obsolete in future versions.

[JimBobSquarePants]

Thanks for that. One of those things we'd have only found out once all our tests break.

[JimBobSquarePants]

I love these kinds of changes. So much better! I'm assuming you want it merged before #2419 ?

[gfoidl]

Yes please merge before the other.
The other probably gets a merge conflict, which I'll resolve then.