Add concurrency option

[niftylettuce]

Similar to p-map, it would be nice to have this built-in rather than have to use p-limit for instance.

[fregante]

How would this even work? Reduce depends on the last value. If it doesn't, then why do you need reduce?

[septatrix]

How would this even work? Reduce depends on the last value. If it doesn't, then why do you need reduce?

One potential use-case would be when the values can be determined asynchronously and independent from each other. Currently this can mostly be archived with pMapIterable, however, that requires a for loop and thus cannot easily be used in e.g. an expression.

[septatrix]

Or in other words: pReduce should be able to accept not only Iterable<Promise|any> but also AsyncIterator<any>. This way one could write pReduce(pMapIterable(values, mapper), reducer, initialVal)