[phobos] [D-Programming-Language/phobos] bc14b1: add is(StringTypeOf!S) for better error msgs

[GitHub via phobos]

  Branch: refs/heads/stable
  Home:   https://github.com/D-Programming-Language/phobos
  Commit: bc14b18f6f50cde0750e15ca8ab54418eacbd565
      https://github.com/D-Programming-Language/phobos/commit/bc14b18f6f50cde0750e15ca8ab54418eacbd565
  Author: Martin Nowak <code at dawg.eu>
  Date:   2015-10-21 (Wed, 21 Oct 2015)

  Changed paths:
    M std/string.d

  Log Message:
  -----------
  add is(StringTypeOf!S) for better error msgs