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 php version check to allow igbinary to be statically built in PHP 8.0+
* Fix bug in out of memory error handling in __sleep, slightly speed up serializing with __sleep.
* Continue serializing remaining properties if a missing property name is returned from __sleep.
* Speed up serializing by optimizing for the case where there is no memory manager override.
When there is a memory manager override, only use that for allocating the string to return.
(benchmarks/serialize-scalar-int.b.php showed a speedup from 0.22 to 0.18 seconds for repeated serialization of a single scalar,
and from 0.186 to 0.180 seconds for benchmarks/serialize-stringarray.b.php for an array of strings)
* Speed up unserializing arrays in php 7.2-8.0 by adding optimized code for finding the hash bucket of
a string/integer key of an array, or creating a placeholder if it does not already exist.