Promises and asynchronous functions

[71104]

They are one honking great idea!

Promises will be available as soon as #65 is fixed because the Promise class can be unmarshalled from the underlying platform normally.

So this bug is mostly about implementing the await statement. E.g.:

let timeout = fn delay ->
  new Promise fn resolve, reject ->
    setTimeout (fn _ -> resolve "hello!") delay in

console.log (await (timeout 1000))  # prints "hello!" after 1 second