[RFC] Bundling ext/simdjson into core

Hi Internals, I think this will be my last proposal for quite some while :) But this time, I'd like to propose bundling the https://github.com/crazyxman/simdjson_php extension with some major modifications. The proposed OO API is included in the description of the PR that I've just created: https://github.com/php/php-src/pull/6551 The main motivation behind this RFC is two-fold: - the underlying simdjson library (https://github.com/simdjson/simdjson) which is used by ext/simdjson provides huge performance gains compared to ext/json (see some benchmark results in the PR) - we can support new use-cases, most notably the so called "on-demand" parsing: https://github.com/simdjson/simdjson/blob/master/doc/ondemand.md (This is not implemented currently) Originally, I planned to include the new API in ext/json, but unfortunately, simdjson is written is C++, so it would make C++ as a hard dependency, which was not the case so far. That's why I opted for creating ext/simdjson.. Please let me know if you have any feedback. Regards: Máté