Add benchmarking to CI

[iluuu1994]

This PR adds benchmarking to CI. It currently runs Symfony Demo, Wordpress and Zend/bench.php (adding more is encouraged in the future) using php-cgi, Valgrind, Opcache (and JIT), and the -T10,1 flag. This flag makes php-cgi execute a warmup request, caching PHP files in shm before running the profiled request. We currently only collect executed CPU instructions. Additional metrics might be useful (e.g. CPU cache misses) but tracking those makes Valgrind factors slower.

The results are then committed to https://github.com/iluuu1994/php-benchmark-data/. This repository should be moved to the php GitHub org. However, I don't have the permission to create new repositories, so somebody would have to do it for me (php/benchmarking-data). Likewise, we've pinned Symfony Demo and Wordpress to exact sources by committing the full source code (including vendors) to separate repositories (https://github.com/iluuu1994/symfony-demo-2.2.3, https://github.com/iluuu1994/wordpress-6.2), (php/benchmarking-symfony-demo-2.2.3 and php/benchmarking-wordpress-6.2, if somebody does it for me). The main reason for this is that changes in master might break the benchmarks, but we can't trivially fix those upstream. Even if we did, pulling in more changes would skew the benchmark, as changes in the libraries may improve or degrade performance for unrelated reasons.

@nielsdos also made a visualization for the collected data, which will show development of performance over time, thank you! https://nielsdos.github.io/php-benchmark-visualisation/

I also experimented with libpfm instead of using Valgrind, which would use hardware to collect metrics and likely be the better solution as we could run benchmarks at real time and collect more metrics and run more benchmarks (see iluuu1994#73). Unfortunately, this requires a dedicated machine to run which would cost $50 monthly upwards on Digital Ocean.