Speed up JSON-AD Serialization

Apparently, in some local tests the server responds faster to JSON-LD or TTL requests (4ms average in dev mode) then JSON-AD (6ms). This is weird, since the JSON-LD and TTL versions fetch all the individual properties, which means they should be far slower than JSON-AD. Something weird is going on, probably inside propvals_to_json_ad_map.

But still... The median response with normal concurrency is 1ms on my laptop, so performance wise, this is not a priority at the moment.

EDIT: Serializing a JSON-AD resource can take up to 1ms for collections. That's way too slow!

I've added a benchmark, and it had serialisation times of 70us for /collections in JSON-AD, 100us for JSON-LD and JSON, and 140 for Turtle. I expected this order, but would still expect JSON-AD to be way faster, since the others need to fetch Properties for every Key, whereas JSON-AD does not have that dependency. JSON-LD serialisation does not even use Store! So I don't get why it's relatively slow.

Start with the existing serialization benchmark and inspect propvals_to_json_ad_map, comparing JSON-AD timings with JSON-LD, JSON, and Turtle for collections. Done means identifying why JSON-AD serialization is relatively slow and recording improved benchmark results or a clear explanation of the remaining cost.