CHANGES:
Spiders will no longer create massive stacks of cocoons under dead comrades. Fixes#2448
Limbs infested with spider eggs will now take longer to burst. When they do burst, the limb is gibbed.
Infested limbs will give out more warning now past a certain stage.
Modified the spider event. The moderate severity one will now no longer spawn nurses, so they can't multiply.
Added a major severity spider event. It spawns more spiders than the moderate severity one along with nurses.
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