Add bulk add/remove/read methods using spans.

[mpartel]

• Added factory methods because overloaded constructors taking (ReadOnlySpan<T> span) and (IEnumerable<T> collection) would be ambiguous given an array.
• This bumps the minimum .NET requirement to .NET Standard 2.1.

[StephenCleary]

I love the span APIs but I can't take the netstandard bump. Can you change this PR to multitarget instead?

[mpartel]

Done, I think (somewhat new to C# outside of Unity)

[mpartel]

Ping. Sorry for piling on more changes afterwards. Should be all done now.

[mpartel]

^ or so I thought :man_facepalming:

[StephenCleary]

Note to self: compare this API with https://github.com/dotnet/runtime/issues/1530

[StephenCleary]

.NET 8.0 just got span overloads for List<T>: InsertRange, AddRange, CopyTo.

https://devblogs.microsoft.com/dotnet/performance-improvements-in-net-8/

[StephenCleary]

Also should include CollectionBuilder in .NET 8.0.