Cache ore entities to reduce calls to `surface.find_entity`

[narc0tiq]

Instead of having the sites retain coordinates, which automatically brings in the need to do a LuaSurface::find_entity call whenever the site wants to recalculate itself, let's have a central ore tracker that feeds out indexes that the sites can retain instead.

This gives some advantages:

• multiple sites storing the same ore entity will have a reference to the same tracker index.
• the tracker can maintain the entities' current amounts, saving find_entity calls on every query.
• the tracker can update entities at a fixed rate (e.g., 50 per tick, which can even be configurable), without having any need for the current iteration state to be stored in the site.

And some disadvantages:

• the ore tracker needs to be notified about entity addition/removal.
• there needs to be a migration from the current setup to the new ore tracker.

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Original content below:

Every tick, we do up to 100 calls to LuaSurface::find_entity for each site that is actively being recounted, and for each force that has that site under active observation. We do these calls to avoid increasing save/load times by storing the actual entities (as we used to do before #26).

However, the call itself probably (!) takes extra time as it crosses the Lua-C++ barrier, whereas if we had the entities stored in a Lua table, we might be able to reduce the access time and thus the on_tick footprint of the mod.

NB: if implemented, this change must be backed up by some actual data; at the very least, set the time-between-site-updates fairly low and track some sites with lots of entities, then check the debug info for whether YARM's tick timing changes with/without the change.

[narc0tiq]

The cache does make a difference:

But it also is not multiplayer-safe... maybe store it in global, I guess.

[narc0tiq]

NB: with the ore tracker as finally designed/developed, we went to a much smoother tick time of 1-1.2ms with the same sites as shown in the image above. However, it also impacted save load time by a few seconds (the example image has sites totalling ~62,000 entities).