Implement Fiber#transfer.

[take-cheeze]

This Fiber#transfer doesn't have compatibility with CRuby's.
On CRuby context has field transferred to check context is transferring though this version doesn't have it.
So transferred context can resume as long as it won't hit normal resume limit.
And transfer to root context is allowed in CRuby but on this implementation it isn't allowed.

What this Fiber#transfer does is that it replaces current running context with transferring context.
Since it use fiber_resume() internally to do it, the limitation of transfer is similar to resume.

[matz]

I am not sure yet how much significant this incompatibility is.
Any opinion?

[bjorndm]

Well, Fiber is not part of the ISO standard anyway, so, in that case, I think cruby compatibility is nice if it can be done without making mruby too heavy. This is just a small change with one extra function really, so I think it's OK add this.

[take-cheeze]

After reading few implementations I've reimplemented CRuby compatible one.
It can now break from nested fiber call.