At least one library (adt) redefines structures with slot names being
uninterned symbols. That means that we lose slot offset if we
reference it by name what leads to load errors if
make-load-form-saving-slots was called. We fix that by handling
structure-object's separately.
allocate-instance is another oddball in the spec when taken with
structure-classes (and it is used in make-load-form-saving-slots). If
there are *some* slots saved then rest must be initialized so object
could be used - we put there NIL without checking for a type. SBCL
tries to find a constructor for a structure (custom protocol) or
errors while CCL initializes slots to NIL. We follow the latter.
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