Issue 12090 - Make std.concurrency compatible with fibers as threads

https://d.puremagic.com/issues/show_bug.cgi?id=12090
There are implications for how they can be scheduled because a Generator must always yield a value.  As it's an input range, if a Generator yielded control to the caller without returning a value... I honestly don't know how to handle this situation.  I think I'd have to throw an Error.