Generate includes from PHP source directly

[iggyvolz]

This PR generates the .h files directly from the PHP source code (committing them to the docker so as not to require a C compiler at runtime), using srcml to extract the relevant symbols. This adds support for PHP 7.4 and 8.1, as well as thread-safe versions (although 7.4 needs some work to get tests passing since the code assumes PHP 8 functionality - not sure if it's worth bothering).

[lisachenko]

Wow! @iggyvolz such an amazing work! It's so pity that I have returned to my project only now, but I really would like to review and merge this PR, just need to allocate some time to check how it works )

I'm going to try to review and test this on weekend and leave my feedback.