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

https://d.puremagic.com/issues/show_bug.cgi?id=12090
Fiber is basic implementation of software thread concept that implements actual context switching. Generator is higher level abstraction which implies that every context switch results in some value produced - and that type of that value is the same every yield.

Generator is just one very specialized use case for fibers.