fetchurl (and derived functions): Support SRI hashes

E.g.

  fetchFromGitHub {
    owner = "NixOS";
    repo = "nix";
    rev = "ad42a784690449873fccb20192bd2150da81c56d";
    hash = "sha256-ZXeadXUJMXV5lSLz6TOBeL/SSOVwQ8ywxU5AFMCnbRU=";
  }