Remove bluebird

[campersau]

Promises in Node.js 10 are significantly faster than before. Bluebird still includes a lot of features like cancellation, iteration methods and warnings that native promises don't. If you are using Bluebird for performance rather than for those - please consider giving native promises a shot and running the benchmarks yourself.

Browsers have also good support for them now and because of #1336 we can use some new APIs on the client like Promise.prototype.finally.

Also using async fs APIs in more places.

[Hirse]

This looks very nice.
I especially like fs.access. 👍