Implement new pop-count

[BNAndras]

I'd like to port this exercise introduced in exercism/problem-specifications#2333.

[ceddlyburge]

That would be great, thanks Andras :)

Do you want any pointers before you start?

This is a recent PR from a new contributor. Its for a concept exercise, which are slightly different to a practice exercises, but it might be useful to look at.
#611

[BNAndras]

Thanks. I’ve ported this exercise for five tracks now so I’ll need to “Elm-ify” my approach including making the test runner happy.

[ceddlyburge]

Woah! You must be getting efficient at porting it now :)