mapPar for collections

[amorfis]

Allows code like in README. Also adds extension to Iterable with .mapParWith.

There is ugly asInstanceOf in the code, any ideas how to do it better are welcome

[rucek]

Woult it make sense to re-use Source.mapPar for this, like:

val res = Source.fromIterable(iterable).mapPar(parallelism)(f)

and then transform res to the expected Iterable using the provided IterableFactory?

[amorfis]

@rucek I tried it, but it's ~3 times slower then directly using semaphores and forks

[adamw]

@amorfis @rucek yes making streams behave better is what I'm working on now :) but with Source.fromIterator there's always an extra fork

[adamw]

Thanks!