[RFC] Normalization-time loaded fields

[rbalicki2]

• Add the ability to make certain imperatively loadable fields load at normalization time, or to execute arbitrary code at normalization time.
• That arbitrary code might:
  • update some other fields in the Isograph store or in an external store
  • start a subscription
• The read out field should no longer have type T, but instead have some arbitrary return type or something containing [T, () => void].

Open question

• Is there anything to be gained by giving the compiler special understanding of fake deferred fields, as opposed to letting the arbitrary code run?