Support .NET Core 2.1 new memory primitives like Memory<T> and Span<T>

[oliverw]

Due to the way NetUV's ReadableBuffer is written, reading the data into a Pipeline requires a temporary copy step. It would be preferable if this could be avoided.

The optimal solution would be of course having NetUV supporting pipelines natively. :)

[StormHub]

@oliverw yep but the catch is that those are net core specific, not net standard 2.0, MSFT is making things available in netstandard 2.1 :)