[Feature request] Add `fs.promises` functions

[CedX]

Dealing with callback-based APIs is harder than Promise-based ones when you need to compose multiple consecutive calls.
So, it would be very cool to add all the functions from the fs Promise API.

EDIT: I know that we can use util.promisify() but we loose type safety as this function returns Rest<Dynamic> ‑> Promise<Dynamic>.

[kevinresol]

Closed by #215