Do not escape expressions with literals only

[emilos]

{2 + 2}

currently gives:

function render(__o, __e) {
  var __t = "";
  __t += __e(2 + 2);
  return __t;
}

but could e.g.:

function render(__o, __e) {
  var __t = "";
  __t += 2 + 2;
  return __t;
}

ideally at some point we'd be able to just get (but out of scope of this task):

function render() {
  return "4";
}