MathSpanUtils: Support a wider set of types.

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E.g. We have MathSpanUtils.Sum(Span), MathFSpanUtils.Sum(Span), and would be good to do the same for Int32 and other native types. E.g. see the Clamp() overloads here:

https://github.com/dotnet/runtime/blob/master/src/libraries/System.Private.CoreLib/src/System/Math.cs

Consider merging MathSpanUtils and MathFSpanUtils, and using partial classes to split the code into multiple files. Perhaps put all of the like method overloads in one file (i.e. all Clamp method in one file).

Also, consider renaming the class to MathSpan or SpanMath.

[colgreen]

Remaining methods to implement:

MathSpan.Int32.cs Sum(), MinMax(), MedianOfSorted(), Clip()

[colgreen]

Done. In 9777bb004fbce5782576812a8e67c076dbd50b1f and d3ccd83afacaa6832b7429cc498ca71e16e99f93,