introduce fiber support

[zhzhxtrrk]

well, thread is not important for mruby, but I do think fiber is a very interesting feature.

[skandhas]

I feel the same.

[matz]

I feel the same. ;-)
But I have too many things to do.

[beoran]

I guess fibers could be emulated in pure Ruby if throw/catch are implemented?

[matz]

I don't think so, because throw cannot be re-entered.

[beoran]

Good point, I hadn't considered that.

[beoran]

Since we do have fibers now, which makes me genuinely excited, let's close this issue. :)