R1:877cb136e856

Add an assocations-like "Edges" framework

Summary:
We have a lot of cases where we store object relationships, but it's all kind of messy and custom. Some particular problems:

- We go to great lengths to enforce order stability in Differential revisions, but the implementation is complex and inelegant.
- Some relationships are stored on-object, so we can't pull the inverses easily. For example, Maniphest shows child tasks but not parent tasks.
- I want to add more of these and don't want to…