Refer to Promise functions via the Promise object in FFI

[thomashoneyman]

Use the functions defined in Web.Promise.Internal via the FFI (resolve, etc.) on master and you'll see an error like:

UnhandledPromiseRejectionWarning: TypeError: #<Object> is not a constructor
  at Object.resolve (<anonymous>)
  at ...

@natefaubion did a brief console session and noticed:

>> var a = Promise.resolve

>> a
function resolve()

>> a(12)
TypeError: Receiver of Promise.resolve call is not a non-null object

and

>> var b = { a: Promise.resolve }

>> b.a(12)
TypeError: b is not a constructor

The adjusted exports in the FFI file avoid these issues.