Async functions + Tests for arraybuffer, dataview and async function

[pjbatista]

• Added support for the ES2017 AsyncFunction type;
• Added tests for ArrayBuffer, DataView and async functions.

[tunnckoCore]

I feel there is too much noise to that PR, but @jonschlinkert is the master here :D I'm basically doing the same thing :D most of the time cuz I'm lazy to do separate PRs for small changes.

And I'm mentioning Jon, because I know what's the feeling to mostly always miss a PR, because of the notification noise. ;d I definitely think to add some GitHub bot to help with that.

[pjbatista]

I mean, I should've added a comment to the commit, but I also cleanup up a few eslint warnings. And I only saw your PR after I already had submitted mine, because asyncs were my main goal too.