Foreach with GetCurrentAsync

[IvanKonov]

Consider the possibility of an asynchronous enumeration of collections. For example, through the support of the GetCurrentAsync method in foreach.

[orthoxerox]

See #43

[DavidArno]

Also, from #840, see https://channel9.msdn.com/Blogs/Seth-Juarez/A-Preview-of-C-8-with-Mads-Torgersen, where Mads discusses possible syntax and functionality for this feature, possibly coming to C# 8.