message: Require explicit &Heap in attribute functions
  Summary: Previous diff (https://github.com/facebookexperimental/starlark-rust/commit/f8d18aceeef4e9e9a1b3697cf76b6411732eb233) explains why this change is done.
diff: @@ -94,7 +94,7 @@ fn render_attr(x: StarAttr) -> TokenStream {
  };
  let let_heap = if let Some(SpecialParam { ident, ty }) = heap {
  - Some(quote_spanned! { span=> let #ident: #ty = heap; })
  + Some(quote_spanned! { span=> let #ident: #ty = __heap; })
  } else {
  None
  };
  @@ -111,7 +111,7 @@ fn render_attr(x: StarAttr) -> TokenStream {
  fn inner<'v>(
  this: starlark::values::Value<'v>,
  #[allow(unused_variables)]
  - heap: &'v starlark::values::Heap,
  + __heap: &'v starlark::values::Heap,
  ) -> #return_type {
  #[allow(unused_variables)]
  let this: #arg = match starlark::values::UnpackValue::unpack_value(this) {
  