Readable interface

[rbalicki2]

Depends on #273.

I think all things that can be read, which might just be fragment references, but could also be other things, should probably implement the following interface:

interface Readable<T> {
  read(): T,
  readButDoNotSuspend(): { kind: 'Pending' } | { kind: 'Value', value: T }
}

and we should only encounter fragment references as items of this interface, not as concrete fragment references.

All APIs (e.g. useResult, FragmentReader, etc.) should accept items of this interface, not fragment references directly.