Make thorns mutation do damage to attacking enemies

Fixes #7447. 

I would appreciate it if someone could provide some explanation as to what the "side" parameter is used for in Creature::deal_damage. I can see that's it's passed on to monster::absorb_hit, but I'm not sure what's occurring with it there. For now I just copied it from the similar code above implementing spine/quill damage. 


I also took a guess as to appropriate damage numbers, and changed the body part/damage type. I figured about half of the damage produced by spines would be right, but let me know if these values need to be rescaled/changed.