C#7.3 Compatibility, .Net Standart 2.0

[blauertee]

This is definetly not ready to be merged.

Target Frameworks have to be Edited and I'm not sure whether you really want to give up on the syntactic shugar of C#9.0.

Also all methods taking byte[] as an argument don't accept Span<byte> in C#7.3 so I just lazily inserted ToArray() everywhere, not sure what this does under the hood but it could lead to serous performance issues.

But I thought this may be useful for someone anyway. :)