Add support for Span<T>

[jackmott]

We should add support for Span, which would allow the use of these functions on stackalloc arrays and other memory sources. If we targeted only .NET 2.0 we could change the function signatures of all methods taking arrays to Spans, but if we want to support .NET Framework and Core 1.0 we need to duplicate every array function, and make a Span version.

As well, performance opportunities may exist if we use stackalloc arrays when appropriate inside our implementations.

[jackmott]

More research on this:
If we merely changed all array methods to take spans, then people using LinqFaster from .NET Framework would take a big performance hit. So we would need to duplicate each function to have array and span versions. Maybe something that could be automated, as it is a lot of grunt work.

[jackmott]

span support added

[BrunoZell]

Any plans to support ReadOnlySpan<T> too? As of now, it looks like the work of duplicating the functions isn't automated.

[Smurf-IV]

Any plans to support ReadOnlySpan<T> too? As of now, it looks like the work of duplicating the functions isn't automated.

See "https://github.com/Smurf-IV/LinqFaster"

[BrunoZell]

#20

[ndrwrbgs]

@BrunoZell if you could open a new issue for that? It seems a bit (related but) tangential to this one
@Smurf-IV if you could break up the review that'd be wonderful :-P Or specific exactly what features to add, so that someone else can add them as smaller reviews that can be reviewed with higher precision/recall (due to better focus area of change)