server/flatpack: Detect and handle cyclic data / share substructure involving all reference types

[cpcallen]

Flatpack (and in particular the methods flatpack.flatten and .unflatten) correctly handle cyclic data and shared substructure involving pointers, but from reading the source for reflect.DeepEqual it's clear that it's possible to encounter cycles that do not involve pointers.

This may not be a problem at the moment because the interpreter makes extensive use of pointers when dealing with cyclic/shared data, but Flatpack should handle all reference types correctly so that we don't need to worry about it elsewhere.

[cpcallen]

Issue moved to flatpack.