Improve fiber test and add some methods to Fiber.

With this pull request:
* The resume of root fiber would be forbidden.
* `double resume` check is fixed.
* `MRB_FIBER_RESUMED` would be replaced with `MRB_FIBER_SUSPENDED`.

In additional it adds following methods:
* `Fiber.root`
  * Returns root fiber.
* `Fiber#==`
  * Compares fiber.
* `Fiber#suspended?`
  * Checks whether fiber is suspended.
  * (*Suspended* means it isn't dead or running.)