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

https://d.puremagic.com/issues/show_bug.cgi?id=12090
I agree that "logical thread" is something most people will intuitively have the right idea about. My concern was mainly about the mix of terminology and the module-level comment claiming things that aren't actually reflected in the implementation (Cid, …). Everything looks good now; thanks for bearing with me.