Fix unexpected free of root fiber object and add correct Fiber#== due to it.

[take-cheeze]

Marking fib field of mrb_context is an alternative of this fix.

[matz]

I am not sure what problem are you going to solve by this PR.
Does creating fiber object each time solve anything?
Is there any program that reproduce the problem?

[take-cheeze]

After running the following script in root context Fiber.current in root context will return MRB_TT_FREE object since fib field of mrb_context won't be marked.(The assertion I added to check_fiber function should fail when checking alive? method)

 root = Fiber.current
 root = nil
 GC.start

Marking fib field seems enough to solve this problem.
Though creating any number of Fiber object from mrb->root_c is allowed(since it won't be freed until mrb_close is called) so I added Fiber#== method.
If you think marking fib is is enough please close this.