message: Slightly clean up the str function
  Summary: No point going via to_str which tries unpacking the string once more - go direct to to_repr.
diff: @@ -870,11 +870,10 @@ pub(crate) fn global_functions(builder: &mut GlobalsBuilder) {
  #[starlark_type(STRING_TYPE)]
  fn str(ref a: Value) -> Value<'v> {
  if a.unpack_str().is_some() {
  - // Special case that can avoid reallocating, but should be equivalent.
  - // Relies on String.to_str giving back exactly what it started with (which it does)
  + // Special case that can avoid reallocating, but is equivalent.
  Ok(a)
  } else {
  - Ok(heap.alloc(a.to_str()))
  + Ok(heap.alloc(a.to_repr()))
  }
  }
  