Made GroupedSequence#each work for async parent sequences

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.