Made GroupedSequence#each work for async parent sequences

[FrozenCow]

GroupedSequence#each was not returning a AsyncHandle when the parent is an async sequence. This patch fixes the issue.

Along with that I've added a simple helper function that could be used in other places to transform (map?) a result. I thought this would be handy in places like https://github.com/dtao/lazy.js/blob/master/lazy.js#L2103.

[dtao]

Haha, it's like you're reading my mind. Literally this morning on the bus ride to work I was patching up some similar issues and had the resolution: I really need to write some sort of generic solution to this problem (of returning either a value or an async handle).

It's a busy day at work, but I'll try to merge your two PRs and push my latest changes this afternoon.

[FrozenCow]

Haha awesome. Take your time, no hurry ;).

It might also be a good idea to use some kind of standard for the AsyncHandle. I haven't used promises much, so I wouldn't know if there's a widely used library for this, but I can imagine there is. (I would imagine functions like transform already being implemented there)

[dtao]

We are on the same page, my friend. #71

[FrozenCow]

I didn't even realize. That's funny :D I'll comment there to keep things on-topic.