Consider integrating simdjson as a potential replacement for rapidjson

[rdw-software]

This library claims to offer even better performance than rapidJSON: https://github.com/simdjson/simdjson

Should evaluate the potential for performance improvements first - comparing to lua-rapidjson, rapidjson + FFI, and dkjson. The latter would only be as a reference point since that's what's most commonly used in the Lua world (presumably)? Having to create and maintain more FFI bindings is a real cost, but then lua-rapidjson seems to be unmaintained and uses the C API...