[DeepClone] Drop SPL "\0" special key from deepclone_hydrate()

Mirrors the C extension change (symfony/php-ext-deepclone#17). ArrayObject,
ArrayIterator and SplObjectStorage all ship __serialize / __unserialize
since PHP 7.4 — callers populate them the standard way:

    $ao = deepclone_hydrate('ArrayObject');
    $ao->__unserialize([ArrayObject::ARRAY_AS_PROPS, ['x' => 1], []]);

The mangled-key resolution path is unchanged; only the SPL fast-path is
removed. Symfony's Hydrator/Instantiator retain BC by translating the
legacy "\0" shape to __unserialize() themselves.

[DeepClone] Collapse $scoped_vars + $mangled_vars into a single $vars…

… param

Adds DEEPCLONE_HYDRATE_MANGLED_VARS flag. $vars is scoped per-class by
default; pass the flag to interpret it as the flat mangled-key array
(array) $obj produces. Footgun guard: NUL-prefixed key in scoped mode
raises ValueError pointing at the missing flag.

[DeepClone] Fix cache poisoning causing spurious NotInstantiableExcep…

…tion

Create tag v1.34.0