feat: Update to std::future::Future

[Marwes]

Can't be merged yet as it depends on async/await being stabilized. Also has one test disabled as https://github.com/facebookincubator/rust-partial-io does not work with tokio-io 0.2.

• Update partial-io and fix test
• Don't depend on the whole of tokio
• Wait for async/await

Closes #184

[badboy]

Hey, thanks for all that work. Indeed, we need to wait for the stabilization, but happy to start reviewing this (and #232).

Do you think we could extract the 2018 edition fixes and land them up front? That would make it easier to also merge other things if they come in already.

[magnet]

Hi, std futures are already stabilized, and async/await stabilization is coming in a month. Several crates in the async/tokio ecosystem have published alpha versions on async/await (e.g hyper, mysql_async, etc) so that we can test them. Would you consider doing the same?

[Marwes]

I think we should jump directly to #232 which requires async await as that improves the API in several ways. I don\t feel like it is ready just yet though as it has a disabled test and some awkward warts which aren't to difficult to fix when I get the time for it (give it a week or two).