Squad formations

[cxong]

Squads #21 were implemented fairly early on, but the AI just has them follow the closest player at a set distance. In practice this forms a conga line, and the problem is that the AI doesn't have awareness of what's in front of them when shooting #295, so they tend to shoot you in the back, which is pretty annoying.

Now that CCD #372 is being implemented, this gives us a new util function to do collision ray casts, which makes it possible to implement formations properly. Perhaps we'll try something like an echelon or line, but we'll see which one works ok with the current hyper-reactive AI.