Promisify async wrapper

[sam-github]

Alternate approach to promisification done by @bajtos in #173

I added more direct support for promise-like fulfill/reject in req_wrap, in the hopes it would be faster if support went deeper into C++.

The benchmarks are noisy on the machines I have access to, but it AFAICT, performance is no better or worse than node, for callback case. Also, no better or worse than #173 for the Promise case.
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There are some open questions about whether direct use of a v8::Promise:Resolver object could be faster, but I'm blocked on that because I can't create one with any slots, so Wrap() can't work on it.

I'm also interested to look at bluebird, which at least one commentator claimed was faster than #173 when it promisifies fs. If so, it should be possible to improve the promise path in pure js.

[vkurchatkin]

whether direct use of a v8::Promise:Resolver object could be faster

I seriously doubt that, because it simple calls some high level js functions.

I don't like the approach where both callbacks and promises could be used. I think there should be separate set of functions that return promises.

[Fishrock123]

I don't like the approach where both callbacks and promises could be used. I think there should be separate set of functions that return promises.

I would prefer it over this situation: fs.thingSync() fs.thing() fs.thingPromise()

[Fishrock123]

Given that promises exemplify the future of async in ES6/7 javascript, we should be looking to the future as having those default returns on async functions IMO. (where it makes sense)

[brendanashworth]

maybe switch to typeof callback === 'function'?

[Fishrock123]

This is interesting, but I'm going to close it out for further discussion on NG as per #11 (comment), and that there isn't momentum at this time. This is a great reference for future discussions though, thanks for making it!