Add Span APIs

[TrueGoric]

As the Span functionality has rocked the .NET world some time ago and is sure to stay, it would be great to see it implemented in SFML.Net.

I am willing to commit my time and submit a PR, however, as this library targets netstandard2.0, this would mean either upgrading to netstandard2.1 (possibly breaking some projects) or dual targeting with an additional dependency on System.Memory.

A question for the maintainers: is it be okay to add such a dependency?

Edit: Possibly related: #174 (comment)

[DemoXinMC]

For the time being, I think it's best we stay on standard 2.0. .NET 5 will potentially be changing a lot of how things are done, and something that requires updating to a new standard would be best if done at the same time as whatever changes are useful for .NET 5

[hawkerm]

It's fairly straight-forward to multi-target for .NET 6 and .NET Standard 2.1 to get those benefits, but then keep support for .NET Standard 2.0 by including the System.Memory package when running there, see this example: https://github.com/CommunityToolkit/dotnet/blob/main/src/CommunityToolkit.Diagnostics/CommunityToolkit.Diagnostics.csproj

There's also projects like https://github.com/Sergio0694/PolySharp to help as well.

[eXpl0it3r]

On the surface such things often seem "straight-forward", the question is then much more, whether it also holds up in the details.

Personally, I'd hold off on a Span API and thus .netstandard2.1 until SFML 3 / SFML.Net 3. However if someone can provide a .netstandard2.0 compatible implementation, we could also consider it for earlier use.

Having just gone through .NET Framework to .NET 6 migration, I personally know how painful it is, when .netstandard2.0 isn't supported. While we don't necessarily have made any compatibility promises, I feel like it could cause a lot of headaches, if SFML.Net 2.x stops being compatible with .netstandard2.0. We'd need some pretty strong reason to break this compatibility.

[hawkerm]

Sorry, I was trying to say you can still use the Span APIs with .NET Standard 2.0 today without having to bump to .NET Standard 2.1.

I was trying to say in fact that you can multi-target, so if someone is running on a newer runtime then they get performance improvements. This is what the .NET Community Toolkit does for their High Performance package.