Additional Using overloads

[sharwell]

Consider providing overloads for TaskBlocks.Using where the body function returns a TResult value instead of a Task<TResult>.

[sharwell]

After examining the source code for the Task Parallel Library in some detail, I determined that the overhead involved with returning CompletedTask.FromResult(result) instead of just result is sufficiently small that this is not a critical issue, even if it does get implemented in a future release.