Present the same Thread object for all fibers within a thread - #1225

The presentation of thread features to a fiber is really tricky. The reason I have not made this change is because thread local variables do not stay with the parent thread, and are instead local to the fiber.

So there's all sorts of nasty conflicts that I have not figure out how to resolve:

1. All fibers under a thread have the same Thread.current.
2. Each fiber has a different set of Thread#[] locals.
3. Our fiber impl uses Ruby-level thread constructs to handle context switching, so they need to have access to the really real Rubythread.
4. Each fiber also needs its own ThreadContext for execution state.

There's a number of conflicting goals here, and so far when I have tried to fix the current broken behavior (1 above), I always end up breaking something else (like 2 or 3 above).

This trivial fix resolves 1, but breaks 2 and potentially 3 depending on how Thread.current is used. I'm afraid this is not enough :-(

@headius thanks for the feedback. Sounds like it needs to be more explicit.
I am keen to push further as a way of learning. Is it OK if I push more changes and questions this way?

@halorgium Sorry for the delay...but yes, it would be great if you could explore this more. I have not been able to find the magic sauce to mimic MRI behavior, since their behavior doesn't seem consistent with either threaded fibers OR lightweight fibers to me.