inclusion: split CandidatePendingAvailability according to the guide (#1413)

* inclusion: split PendingAvailability storage into descriptor and commitments

* inclusion: fix tests

* implementers-guide: update CandidatePendingAvailability type

* inclusion: simplify process_candidates a bit

* implementers-guide: more updates to the inclusion module

* inclusion: fix copy-paste errors in tests

* inclusion: revert some of the changes

* inclusion: lazy commitments loading and a test

* guide: revert enact_candidate changes

* inclusion: test process_bitfield for no commitments

* Grammar

Co-authored-by: Robert Habermeier <[email protected]>

Co-authored-by: Robert Habermeier <[email protected]>