[ghc-steering-committee] #570: Native string interpolation syntax (round two)

Dear committee, In #570 <https://github.com/ghc-proposals/ghc-proposals/pull/570>, Brandon Chinn proposes adding native string interpolation syntax to GHC, along the lines of s"a ${x + 1} b", catching up with and even exceeding similar features in popular programming languages. I think this is an important proposal and I recommend that we accept it. Since the previous submission, Brandon settled on a coherent and simple interaction with -XOverloadedStrings. Although the proposed interpolation API seems more ad-hoc than necessary, I did not find a way to simplify it any further. Cheers, Sebastian