Add `IEnumerable<CFItem> EnumerateChildren()` to CFStorage

[tstenner]

This PR adds an EnumerateChildren() method and rewrites VisitEntries() to use it in the background.

This allows both enumerate via foreach as well as LINQ as wished for in #58.

Examples, adapted from the unit tests:

// total number of storages
stg.EnumerateChildren(true).Where(e => e.IsStorage).Count();
stg.EnumerateChildren(false).Where(e => e.Name.Contains("Stream")).Count();
foreach(var item in stg.EnumerateChildren(false))
                if (item.Name == "MyStream") System.Console.Out.WriteLine(item.Name);