Contacts should be networked

[metalgearsloth]

At least a small subset of data. The issue is stuff like steptrigger, conveyors, etc. tracking entities on their own and duplicating data, plus you can't rely upon getcontactingentities in a shared context when handling prediction.

At a minimum we should network:

• Fixture IDs
• EntityUids

We can't store these on the fixture directly because it has a bunch of extra data and the delta would be messy.