Add performance tests to PR actions

It is important to maintain performance parity to std::span (at the very least when Safe Buffers are enabled). In order to track this, we should have performance tests between std::span and gsl::span for the latest standard library implementation across Clang, GCC, and MSVC.

@galenelias put together small benchmark comparing the two implementations on various workloads. They have been attached for convenience and provide a good starting point.

span_bench.zip