Switch to futures 0.3

[piegamesde]

Depending on futures 0.1 is causing me headaches, because the traits mismatch (I have two different futures versions in my dependency tree).

[koba-e964]

We have not decided whether we should switch to futures:0.3. We don't have any clear prospect for this matter right now, so for now there are solutions for you (and anyone who uses fibers-rs) to:

• stop using fibers-rs and migrate to tokio or some other libarires, or
• rewrite fibers-rs so that it depends on futures:0.3 (If you do this, I rewrote halfway: https://github.com/dwango/fibers-rs/tree/feature/tokio-1.0. This can also be a staging point when you migrate from fibers-rs to tokio. This thing uses tokio internally. No docs or whatever to help users so it can cause headaches though)