Fix bitfield signing (#1466)

* Apply suggestions from #1364 code review

- use CoreState, not CoreOccupied
- query for availability chunks, not the whole PoV
- create a stub `fn availability_cores`

* link to issue documenting unimplemented

* implement get_availability_cores by adding a new runtime api request

* back out an unrelated change properly part of #1404

* av-store: handle QueryChunkAvailability

* simplify QueryDataAvailability

* remove extraneous whitespace

* compact primitive imports