lazily initialise Object::Pad fields at first read

 Object::Pad::FieldAttr::LazyInit provides a third-party field attribute for
 Object::Pad-based classes, which declares that the field it is attached to
 has a lazy initialisation method, which will be called the first time the
 field's value is read from.
 .
 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-lazyinit-perl
 following the renaming and deprecation of Object::Pad::SlotAttr::LazyInit
 upstream.