php 8.1 is more strict, and raises some deprecation notices with existing protobuf code. Not all of the
deprecations can be fixed without dropping support for php7.x (eg, the 'mixed' type doesn't appear until
8.1, and union types until 8.0, but as an interim solution the 'ReturnTypeWillChange' attribute can be
used to suppress the notices. In passing, also be explicit about a cast from float to int in 'zigZagEncode64'
which 8.1 also complains about when running tests.

bcmath (specifically, the bccomp function) is internally required, and tests fail if it's not available

With guidance from Remi Collet, use ZEND_BEGIN_ARG_WITH_TENTATIVE_RETURN_TYPE_EX macro, and use a conditional to fake that macro
for earlier php versions. Tested on 8.1 and 7.4, and deprecation notices gone plus all tests pass

The macro ZEND_BEGIN_ARG_WITH_RETURN_TYPE_INFO_EX changed in 7.2, so it cannot be used in a compatible
way with earlier versions

instead of using the void return type, add more ReturnTypeWillChange annotations

I think it _should_ be required, but a test (linux, 32bit, 7.0-zts) is choking
on composer install, so putting things back to how I found them