Make std.concurrency compatible with fibers

https://d.puremagic.com/issues/show_bug.cgi?id=12090

With this commit fibers can have own message boxes and act similar
to threads in that regard.
Add Generator to std.concurrency

Generator is an extension to a Fiber that yields return values of specific type
Merge pull request #1910 from complexmath/addConcurrencyScheduler

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