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.

* Use PHP's shared empty array instance when unserializing empty arrays in php 7.3+.
(helps slightly with memory usage when repeatedly unserializing,
when removing elements from arrays before unserializing them,
or when serializing values including an empty array that was unserialized)
* Emit a deprecation notice when serializing resources.
PHP itself is converting many resources to objects that throw an Error on serialization attempts.
Continue to represent resources as null in the serialized data.
* Fix memory management bug when unserializing invalid data (duplicate properties in objects (e.g. from `__sleep`) or duplicate fields in arrays (impossible for valid data)).
* Speed up calls to `__serialize`/`__unserialize` in php 8.0+.
* Fix error messages for unserialize_callback_func: make messages properly refer to the autoload function.
* Optimize unserializing alternative names for private/protected constants that were previously public.