Added support for PHP 8.1 (currently in RC1) to the C extension

[haberman]

This switches to new interfaces for spl_ce_ArrayAccess and spl_ce_Countable (zend_ce_arrayaccess and zend_ce_countable, respectively). It polyfills zend_ce_countable for old versions of PHP.

There is also a small fix for the tests to fix the following error:

1) GeneratedClassTest::testMapFieldViaArray
Implicit conversion from float 6.1 to int loses precision

In PHP float array keys are truncated to integer, and this has nothing to do with protobuf.

The pure PHP library may require further fixes to support 8.1. I was not able to resolve those issues at this time.

Fixes: #8711