Map should only produce array; add mapValues and mapKeys

[megawac]

There was another issue/pr? for this I made sometime last year but currently map can produce either an object or array because of this line:

https://github.com/caolan/async/blob/master/lib/internal/map.js#L11

I would prefer we follow underscore/lodash lead and produce only arrays from map and then add mapValues and mapKeys for mapping objects (#1133).

This is one of the last changes I want to get into v2. Thoughts @aearly

[aearly]

Yes, I like the idea -- one last breaking change that fixes an inconsistency. It's odd that map returns an array for Maps and Sets, but not for objects.

What would the iteratee signatures look like? mapValues(obj, function (value, key, cb) {...}, done) / mapKeys(obj, function (value, key, cb) {...}, done)?

Would mapValues/mapKeys work with Maps too?

This would also come in Series and Limit flavors too.

[aearly]

Anyone working on this?