Add utility function to convert GraphQL spec incremental response for @defer/@stream fragments so that they work with the expected shape from Relay.

Refactor Defer response converter so that the unsafe magic JS external function is just the object merge. Also implement a type-safe converter using object types, though it may not be necessary in the future as this problem should be resolved if Relay starts conforming to spec.

Fix type errors for the interface file.
Reimplement to avoid use of Array.filterMap which is included in Rescript Core.