JSONize mending modifier from mutations

[anothersimulacrum]

Summary

SUMMARY: None

Purpose of change

The effect mutations have on limb mending was hardcoded in a if/else ladder. Yuck! Also, some of the slow healer mutations were not having an impact on mending speed, so add that.

Describe the solution

Add mending_modifer value to mutations, of type float.
Add code to load this, and replace the if/else ladder with multiplying times this multiplier.
Add this value to mutations.

Testing

Break an arm, and debug in the reptilian healing mutation. Sleep, and notice that your arm mends very quickly, as it should.