Weapon launch curves

[Kestrellius]

Adds a modular curve set that governs the behavior of weapon creation -- things like fire rate and burst shot count.

This is kind of messy, and if anyone has ideas for how to make it less messy I'm all ears, but I think it's just going to have to be this way -- weapon firing behavior is spread over like five different functions in three separate places, not even counting the edge cases. This messiness means that every input for these curves needs to be passed around a lot, so for now I've kept the inputs limited to the ones that I immediately need for my specific use-cases.

Important limitations of these curves at present:

• They don't work for beams (because those are a different type of thing that would need its own curveset), subspace missiles (because I couldn't be bothered), or weapons spawned as children or generated by SEXP (because the inputs are undefined). In all those cases except beams, both inputs will always be zero.
• The outputs include a max velocity multiplier and a lifetime multiplier, both of which may affect when the weapon can be fired and how it is best aimed. The AI has no idea that these multipliers exist; it will behave as though the base tabled values are in force. For the moment, I think this is the best option. I looked at trying to have the AI take it into account, and things like max velocity are referenced in so many places that it doesn't look very feasible to me -- even setting aside the implementation hassle, I have no idea what effect it'd have on performance. Further, these curves permit things like weapons whose velocity changes based on the distance to the target -- if we imagine trying to hit something like a fighter with one of those, even a human would have a lot of difficulty performing that logic. I think keeping it simple and allowing the modder to control the AI behavior using the tabled values will work best.

Also, I attempted to add multi support. If I'm understanding correctly I think it will work, but I'm not ultimately very familiar with the multi code at all. Additionally, my method requires adding an extra float to each turret_fired and flak_fired packet, and I have no idea if that's an acceptable cost given that the functions are going to the trouble of squishing ints down into shorts.

[BMagnu]

Small things.
Also, someone more familiar with multi code should take a look here.

[BMagnu]

This should take a const ref for the curve data

[BMagnu]

Here too

[BMagnu]

This is an API change and thus needs to increment the multi version number at some point in this PR.

[BMagnu]

const ref as well

[BMagnu]

same here

[JohnAFernandez]

The multi related changes do look safe, but we are going to need to increment the multi version, which is in multi.h. Ping me on discord if you need more detailed instructions.