message: Make SmallMap::fields field private
  Summary: No reason to be public.
diff: @@ -108,7 +108,7 @@ starlark_complex_value!(pub Struct<'v>);
  #[repr(C)]
  pub struct StructGen<'v, V: ValueLike<'v>> {
  /// The fields in a struct.
  - pub fields: SmallMap<V::String, V>,
  + fields: SmallMap<V::String, V>,
  }
  unsafe impl<'v> Coerce<StructGen<'v, Value<'v>>> for StructGen<'static, FrozenValue> {}
  