Mutation tweaking (mostly Ursine)

Long story short, yeah, Ursine's got most of its stuff post-Threshold so it's tougher to get there than it really needs to be.  I added two more mutations and tagged the Claws as Ursine as well, so that helped a bit.  

However, I ended up having to pull the random-mutation from mutagen rolling a post-Thresh, as it was making test-breaches impossible.  Net result, Bears get better damage-soaking (I heard you when you said they couldn't tank well enough--if that's still not enough, let me know), marginal improvement to swim speed, and everyone with body-size upgrades gets bonuses against poison.

Other tweaks: Hyper-Metabolism now immediately and silently digests tapeworms: if you're processing food quickly enough that you can eat to heal your wounds, the tapeworm hasn't got a chance in that gut.

Fixed #6895 as well.