High resolution timer function

Hi, I would like to propose a function for high resolution monotonic timing. There was discussions about this before and a PR https://github.com/php/php-src/pull/2368 which has issues and was abandoned. I've filed https://github.com/php/php-src/pull/2976 with some reworked implementation. A monotonic timer can be usable in several situations besides benchmarking. Having a simple functionality like this in the core should be a useful addition. The current approach is a function returning array of [seconds, nanoseconds] and optionally returning full nanosecond number as int on 64-bit or float on 32-bit. The first way is the most portable. Quite a few platforms are already supported by the current implementation. IMHO it should be fine to have a function like this in the core, perhaps also a few helper functions could be useful, too. I would like to pursue 7.3 with this. Please lets check for any concerns in general or with implementation, naming, etc. Regards Anatol