Use Span<T> with fixed fields

[AArnott]

From @AArnott on February 9, 2019 18:37

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.

Copied from original issue: dotnet/roslyn#33272

[tannergooding]

This is very similar to the already championed fixed-sized buffers proposal: #1314 (which proposes supporting fixed-sized buffers of any type and relying on ref returning indexers).

[YairHalberstadt]

Closing as effectively subsumed by #1314, and decided in #1502 that it's not currently under consideration.