Reduced chance of combat skill rust for post-threshold predators

The reduction in combat skill rust rate for various tiers is equal to the existing learning bonus they get, calculated as follows (all skill gain is unchanged, numbers here are just to show where I came up with 25/50/67):

| Mutation | Learn bonus | Learn rate | Bonus/total |
| -------- | -----------:| ----------:| -----------:|
| PRED2    |         33% |       133% |         25% |
| PRED3    |        100% |       200% |         50% |
| PRED4    |        200% |       300% |         67% |

So this gives PRED2 a 25% chance to avoid combat skill rust, PRED3 a 50% chance to avoid it, and PRED4 a 67% chance to avoid it.

Going by the comments in mutation.cpp about the PRED3/PRED4 int penalties, it's not so much that they're becoming stupid as it is their brains are focusing on hunting and paying less attention to those other trivial things like using machines or building things.

Given that, it makes sense that they'd be more likely to remember that which they're specifically optimized for.

Note that anyone with PRED3 or PRED4 is taking a 1-3 point int penalty (and therefore rust penalty to everything in int-based rust modes) and that half of the PRED4 categories also include forgetful, so this isn't as significant a boost as it might appear at first glance.

When they pass this check, it very rarely also prints a message that gives (one point, just for fun) stim. This averages once every 8/4/3 hours for PRED2/PRED3/PRED4.