Revert MRB_FIBER_RESUMED as MRB_FIBER_RESUMING.

After looking back I found what I should fix was [this error](http://melpon.org/wandbox/permlink/yYkwRiaqge0OuHaB) [(14 days ago mruby)](https://github.com/mruby/mruby/tree/47d1701fb4d94c8710c4045372c6aa9dd4f5e909) and I shouldn't remove `MRB_FIBER_RESUMED`.
Just adding `MRB_FIBER_SUSPENDED` and adding check `c->status == MRB_FIBER_RUNNING` was enough.

(`MRB_FIBER_RESUMED` Fiber can be detected by checking its context is any of the element of the linked list from `mrb->c` to `mrb->root_c` that is connected with `prev` field of `mrb_context`. So if I added a check to linked list, `MRB_FIBER_RESUMED` can be truly removed.)

I renamed `MRB_FIBER_RESUMED` to `MRB_FIBER_RESUMING` since it's similar to `MRB_FIBER_RUNNING`.

Root fiber resume check can be done without modification since root fiber is always `MRB_FIBER_RUNNING` or `MRB_FIBER_RESUMED`.
I've added to check revert is working correctly.