We are slower than PMJSON

[tyrone-sudeium]

On my computer, we are something like 1.26x slower than PMJSON when running the JSON Core performance test.

We lost some of our performance in the Swift 3 migration, and with the addition of the accessor helpers. There's definitely some potential gains to be made with just a little time spent in the time profiler.

My latest raw numbers: PMJSON: 8.964 seconds JSONCore: 11.262 seconds NSJSONSerialization: 22.840 seconds