Use fs.promises where possible

[benjamingr]

Node now ships with an fs promise API, it would be cool if mz would detect if that API is available (optionally?) and use it if it's there.

[RyanZim]

As of Node v10.1, usage of fs.promises raises an ExperementalWarning in the console, I don't think we should use that until it's stable.

[benjamingr]

Fair enough - feel free to close or keep open as a tracking issue - we intend to make it non-experimental as soon as we've gathered enough user feedback about it :)

Thanks for the quick response.

[RyanZim]

Not on the team, just commenting here.

[LinusU]

What @RyanZim said 😄

Yeah, we really don't want to trigger ExperementalWarning for our users, so let's wait until it's stable ☺️

[RyanZim]

@LinusU Speaking of which, #63 should be fixed 😉

[ExE-Boss]

Usage of fs.promises would be a breaking change, since fs.promises returns an object for callbacks returning more than one item, whereas mz returns an array in those cases.

Compare the return type of fs.promises.write(…) to that of mz/fs.write(…).