Evaluate openresty luajit2 API extensions

[HaroldCindy]

https://github.com/openresty/luajit2 some of the table functions and the thread recycling code would be useful, especially whereas we're creating and deleting a lua thread for each handler invocation.

[tapple]

lljson already has the isarray test; there wouldn't be much overhead exposing it. I can't efficiently emulate it using lljson though:

function isarray(t)
     valid, result = pcall(lljson.encode, t, {replacer = function() return "" end})
    return valid and result:sub(1, 1) == "["
end

I can't currently think of an application for table.isarray though

[tapple]

by far my favorite of those is if table.isempty(t). It's much clearer than the equivilant if not next(t). and a clearer conversion for the lsl code if (t == []), which I have a bit of. Some of those end up turning into a nil check after refactor, but not all. Every time I encounter if not next(t), I have to stop and think what it's testing. Maybe it will grow on me, but it hasn't yet. For my own sanity, I ended up converting the array ones (most so far) again to if #t > 0

The only use cases I can think of for table.countkeys would be non-critical:

1. printing debug statistics in hovertext/chat (I plan to use a dict as a fast unordered inter-coroutine queue soon)
2. Passing it as an argument to table.create when converting a dict to array. (I don't know how often this would come up, as I don't yet have many dicts, still being early in my lsl conversion)