Epic: overhaul polymorphism
I want to turn each polymorphic position into a boundary (so the type of a parent is irrelevant, it's just the data that matters; and it can't be optimized through) and then run the polymorphism in a subplan (something we don't have yet), and re-group each time we hit another polymorphic position. Ideally we'll also make it so that we can break off planning at the boundaries and come back to it later when it's needed; but that can be a Grafast V2 feature if it turns out to be too hard to implement.

```mermaid
flowchart TD
subgraph before
allPets-->Cat
allPets-->Dog
allPets-->Fish
Cat-->C1[owner]
C1-->C2[Business]
C1-->C3[Person]
C2-->CZ4[firstPet]
CZ4-->CZ41[Cat]
CZ4-->CZ42[Dog]
CZ4-->CZ43[Fish]
C3-->C4[firstPet]
C4-->C41[Cat]
C4-->C42[Dog]
C4-->C43[Fish]
Dog-->D1[owner]
D1-->D2[Business]
D1-->D3[Person]
D2-->DZ4[firstPet]
DZ4-->DZ41[Cat]
DZ4-->DZ42[Dog]
DZ4-->DZ43[Fish]
D3-->D4[firstPet]
D4-->D41[Cat]
D4-->D42[Dog]
D4-->D43[Fish]
Fish-->F1[owner]
F1-->F2[Business]
F1-->F3[Person]
F2-->FZ4[firstPet]
FZ4-->FZ41[Cat]
FZ4-->FZ42[Dog]
FZ4-->FZ43[Fish]
F3-->F4[firstPet]
F4-->F41[Cat]
F4-->F42[Dog]
F4-->F43[Fish]

end
```
```mermaid
flowchart TD
subgraph after
_allPets-->_Cat[Cat]
_allPets-->_Dog[Dog]
_allPets-->_Fish[Fish]
_Cat-->owner
_Dog-->owner
_Fish-->owner
owner-->Business
owner-->Person
Person-->firstPet
Business-->firstPet
firstPet-->_FP1[Cat]
firstPet-->_FP2[Dog]
firstPet-->_FP3[Fish]
end
```