Span<T> support

[AlgorithmsAreCool]

Now that System.Memory is available, would you be willing to accept some PRs adding Span overloads to get the API closer to 0-copy?

[vpenades]

I'm working on a project that would require compressing huge amounts of data, and I'm trying to avoid allocating as many buffers as possible, so using Span as source and targets for reading and writing would be great to improve performance