Add mkString_ that just takes separator string

[matthughes]

Cats provides a Show-compatible version of mkString but it requires all three parameters: prefix, separator, suffix. The std library also provides a mkString that just takes a separator string; this is what I use 90% of the time.

Can we add this? I'd be happy to PR.

[calvinbrown085]

Hi @matthughes did you end up working on this?

[matthughes]

@calvinbrown085 I have not. I wanted to see if there was interest before doing the PR as it's pretty simple. Really just adding a new method that delegates to the form:

def _mkString(separator: String): String = 
  _mkString("", separator, "")

[kailuowang]

@calvinbrown085 I think this would be a good addition. And it's rather simple, the code you wrote and a doctest should do.

[calvinbrown085]

@kailuowang @matthughes Feel free to take this one since you originally opened the issue :)

[dwijnand]

Fixed by #2709