igbinary extension

Igbinary is a drop in replacement for the standard php serializer. Instead of
the time and space consuming textual representation used by PHP's serialize(), igbinary stores php data
structures in a compact binary form. Savings are significant when using
memcached or similar memory based storages for serialized data.

* Update test expectations for php 8.2.0-dev. Add `#[AllowDynamicProperties]` Attribute to some tests to avoid notices.
* In php 8.1+, make igbinary_unserialize check to see if an equivalent interned string already exists when unserializing object property names, array keys, and class names
and use that instead of creating a brand new string.
(This deliberately doesn't create a new interned string if one doesn't already exist.)
(Before this change, igbinary would deduplicate strings when serializing, but would not check if strings were interned by PHP itself when unserializing)
* Avoid debug build assertion failure for `HT_ASSERT_RC1` the same way as PHP's unserialize - this is a case where ostensibly there are no other references to the array being unserialized.