R1:674388ce6aee

Prepare DestructionEngine to be modularized

Summary:
Ref T9979. The general shape of "engine" code feels pretty good, and I plan to move indexing to be more in line with other modern engines, with the ultimate goal of supporting subprojects (T10010) and several intermediate goals.

Before moving indexing, clean up Destruction, since some of the new indexes will need destruction hooks and destruction currently has a lot of `instanceof` stuff that should be easy to fix by applying more modern approaches.