Use Span<T> with fixed fields

[AArnott]

C# structs can have inline arrays like this:

unsafe struct Some
{
   private fixed byte something[4];
}

But then something is typed as byte*. With Span<T> we should have a safe alternative. Something like:

ref struct Some
{
    private fixed Span<byte> something[4];
}

This removes the unsafe keyword and lets us use Span<T> which embeds the length, making it safer.

[AArnott]

This issue was moved to dotnet/csharplang#2208