Improve Future Compatibility With Data Fixers On Backpack Contents

[williewillus]

Datafixers will not run on the stacks held inside backpacks, meaning any stack format changes made by Mojang or other modders will not be run.

Details: https://www.reddit.com/r/feedthebeast/comments/7fbqfw/psa_modders_are_you_calling_datafixers_on_your/

[gr8pefish]

Add Data Fixers for 1.12