Implement `IComparable<T?>` and `IEquatable<T?>`

[brcrista]

According to the guidance at https://github.com/dotnet/runtime/blob/master/docs/coding-guidelines/api-guidelines/nullability.md#interface-implementations

IComparable<T>

• [x] Token<T>

IEquatable<T>

• [x] Either
• [x] Of<T>
• [x] Optional (value type)
• [x] Result
• [x] Token<T>
• [x] Unit (value type)

[brcrista]

I tested this out and it works as you'd expect. IEquatable<T> looks like:

public interface IEquatable<T>
{
    bool Equals(T? obj);
}

So, IEquatable<T?> looks exactly the same, and you can implement it without additional modifications. But, if you have a method that takes a parameter of type IEquatable<T?> and you only implement IEquatable<T>, you'll get a warning.

On the other hand, you can assign an IEquatable<T?> to an IEquatable<T> just fine.