• 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.