Make Js.Promise optional

[ulrikstrid]

My long term goal for this is that I want this to be useable in native as well.

For that to happen we need to make it possible to use lwt or something like it for the async handling as well as Promises when compiling to JS.

To do this we need to create a abstract async handler that can be fulfilled with either Promises or something else from the native side.

[wokalski]

Or just functorize it 🙃