declare Object::Pad fields read-only after construction

 Object::Pad::FieldAttr::Final provides a third-party field attribute for
 Object::Pad-based classes, which declares that the field it is attached to
 shall be set as read-only when the constructor returns, disallowing further
 modification to it.
 .
 WARNING The ability for Object::Pad to take third-party field attributes is
 still new and highly experimental, and subject to much API change in future.
 As a result, this module should be considered equally experimental.
 .
 This package is a replacement for libobject-pad-slotattr-final-perl following
 the renaming and deprecation of Object::Pad::SlotAttr::Final upstream.