Null character escape not encoded correctly in most backends

[robin-aws]

This program is a doozy, it ONLY runs without crashing on C#:

method Main() {
  var s := "\03";
  assert s[0] == 0 as char;
  assert s[1] == '3';

  expect s[0] == 0 as char;
  expect s[1] == '3';
}

Most backends are not escaping the \0 escape properly, and hence trying to interpret \03 an octal escape. Go attempts to handle this by padding to \003 which is a valid Go escape sequence, but in so doing changes the value of the string!