Consider removing `CompletionSourceList`

[jonsequitur]

    // IReadOnlyList, yet it has an Add.  Why isn't it IList?
    // Of course, there's also
    // Guideline: DO use the “Collection” suffix in names of types implementing IEnumerable (or any of its descendants) and representing a list of items.
    // So "CompletionSourceCollection" is the name the guidelines suggest.  (It doesn't get the exception of "if you're just being a general data structure and explaining the kind of data structure" that List has)
    // Why doesn't this look like other collections? (Count property, etc)?
    public class CompletionSourceList : IEnumerable<ICompletionSource>, IReadOnlyCollection<ICompletionSource>, IReadOnlyList<ICompletionSource>, IEnumerable 
    {
        public CompletionSourceList();
        public int Count { get; }
        public ICompletionSource Item { get; }
        public void Add(ICompletionSource source);
        public void Clear();
        public IEnumerator<ICompletionSource> GetEnumerator();
    }