Add IsString instance for ShortText newtypes

[phadej]

Resolve #4224

[mention-bot]

@phadej, thanks for your PR! By analyzing the history of the files in this pull request, we identified @dcoutts, @ezyang and @hvr to be potential reviewers.

[23Skidoo]

LGTM.

[23Skidoo]

May be worth copying splitOn from Data.List.Split to D.S.Utils.

[phadej]

Let's do it later. This commit just moves as little code as possible

[23Skidoo]

Sure.

[phadej]

Now i remember, splitOn is quite general reusing other combinators in split. Also this is less general splitOn' :: a -> [a] -> [[a]], where Data.List.Split.splitOn :: [a] -> [a] -> [[a]]

[23Skidoo]

You're right, this is more like Data.List.wordsBy.

[ezyang]

OK!

[23Skidoo]

Travis failure is genuine:

Distribution/Utils/ShortText.hs:20:1: Warning:
    The import of ‘Data.String’ is redundant
      except perhaps to import instances from ‘Data.String’
    To import instances alone, use: import Data.String()
<no location info>: 
Failing due to -Werror.

[phadej]

new try

[phadej]

Ah. cannnot bump optparse-applicative because of haskell/hackage-security#179. Will wait for it.